Comments View Full Screen Brake technology, just like suspension technology and fuel-system technology, has come a long way in recent years. What began in the '60s as a serious attempt to provide adequate braking for performance cars has ended in an industry where brakes range from supremely adequate to downright phenomenal. The introduction of components like carbon fiber, sintered metal and lightweight steel, along with the adoption of ABS, have all contributed to reduced stopping distances and generally safer vehicles (though ABS continues to provide controversy). One of the first steps taken to improve braking came in the early '70s when manufacturers, on a widespread scale, switched from drum to disc brakes. Since the majority of a vehicle's stopping power is contained in the front wheels, only the front brakes were upgraded to disc during much of this period. Since then, many manufacturers have adopted four-wheel disc brakes on their high-end and performance models as well as their low-line economy cars. Occasionally, however, as in the case of the 1999 Mazda Protege's, a manufacturer will revert from a previous four-wheel disc setup to drum brakes for the rear of the car in order to cut both production costs and purchase price.

Why are disc brakes better than drum and how much, if any, loss of braking occurs when using rear drum brakes on a modern car? That's what we're going to find out in this edition of Tech Center. Friction and Heat Before you can appreciate the difference between drum and disc brakes, you have to understand the common principles that both systems use when stopping a car: friction and heat. By applying resistance, or friction, to a turning wheel, a vehicle's brakes cause the wheel to slow down and eventually stop, creating heat as a byproduct. The rate at which a wheel can be slowed depends on several factors including vehicle weight, braking force and total braking surface area. It also depends heavily on how well a brake system converts wheel movement into heat (by way of friction) and, subsequently, how quickly this heat is removed from the brake components. This is where the difference between drum brakes and disc brakes becomes pronounced. Drum Brakes Early automotive brake systems, after the era of hand levers of course, used a drum design at all four wheels. They were called drum brakes because the components were housed in a round drum that rotated along with the wheel.

Inside was a set of shoes that, when the brake pedal was pressed, would force the shoes against the drum and slow the wheel. Fluid was used to transfer the movement of the brake pedal into the movement of the brake shoes, while the shoes themselves were made of a heat-resistant friction material similar to that used on clutch plates. This basic design proved capable under most circumstances, but it had one major flaw. Under high braking conditions, like descending a steep hill with a heavy load or repeated high-speed slow downs, drum brakes would often fade and lose effectiveness. Usually this fading was the result of too much heat build-up within the drum. Remember that the principle of braking involves turning kinetic energy (wheel movement) into thermal energy (heat). For this reason, drum brakes can only operate as long as they can absorb the heat generated by slowing a vehicle's wheels. Once the brake components themselves become saturated with heat, they lose the ability to halt a vehicle, which can be somewhat disconcerting to the vehicle's operator. Disc Brakes Though disc brakes rely on the same basic principles to slow a vehicle (friction and heat), their design is far superior to that of drum brakes.

Instead of housing the major components within a metal drum, disc brakes use a slim rotor and small caliper to halt wheel movement. Within the caliper are two brake pads, one on each side of the rotor, that clamp together when the brake pedal is pressed. Once again, fluid is used to transfer the movement of the brake pedal into the movement of the brake pads. But unlike drum brakes, which allow heat to build up inside the drum during heavy braking, the rotor used in disc brakes is fully exposed to outside air. This exposure works to constantly cool the rotor, greatly reducing its tendency to overheat or cause fading. Not surprisingly, it was under racing circumstances that the weaknesses of drum brakes and the strengths of disc brakes were first illustrated. Racers with disc brake systems could carry their speed "deeper" into a corner and apply greater braking force at the last possible second without overheating the components. Eventually, as with so many other automotive advances, this technology filtered down to the cars driven by everyday people on public roads. Drum vs. Disc: Today In today's automotive pantheon, it's not uncommon to find four-wheel disc brakes as standard equipment on medium-priced, non performance-oriented models.

The majority of new vehicles, however, continue to utilize a front-disc/rear-drum brake setup. What does this say about the current state of braking systems? Are these manufacturers sacrificing vehicle safety in order to save a few bucks by installing disc brakes on only the front wheels? While a "yes" answer would certainly be great for increasing Town Hall traffic, the truth is that today's disc/drum setups are completely adequate for the majority of new cars. Remember that both disc and drum brake design has been vastly improved in the last 20 years. In fact, the current rear drum brake systems on today's cars would provide better stopping performance then the front disc setups of the '70s. And today's front disc brakes are truly exceptional in terms of stopping power. Combined with the fact that between 60 and 90 percent of a vehicle's stopping power comes from the front wheels, it's clear that a well-designed, modern drum brake is all that's required for most rear wheel brake duty. High performance cars like the Viper, 911 and Corvette can justify a four-wheel disc brake system, especially if their owners participate in some form of sanctioned racing activity on the weekends.

The rest of us get more of a benefit from the lower cost of drum brakes. Expecting every vehicle built today to come with four-wheel disc brakes would require an across-the-board increase in purchase price, and that could stop new car buyers much quicker than any brake system. To find a dealership that knows how to treat shoppers right, please visit Edmunds.com's Dealer Ratings and Reviews . Disc Brakes and Drum Brakes Explained! Search by Make Model Year, Product Type, Part Number, or Brand... home / articles / Disc Brakes and Drum Brakes Explained! Disc Brakes and Drum Brakes Explained! by Michael Grayen on October 17, 2014 If you have owned a car or truck for more than a few months, you are undoubtedly aware that your vehicle's brakes occasionally need replacing. Let's face it, we can be hard on our brakes. Highway speeds, stop-and-go traffic, and heavy cargo loads are some of the more significant factors which increase brake wear. Brakes are considered a normal wear-and-tear item and sooner or later (sooner for the heavy-footed among us) brake parts always need replacing.

But what is your mechanic referring to when he tells you that you need "pads AND rotors"? Or tells you that, "you need front pads, rear shoes, and one caliper is sticking"? Do you know what kind of brake system is on your ride? More importantly, if you're purchasing the parts yourself, do you know the difference between "pads" and "shoes"? Between "rotors", "discs" and "drums"? Read on and find out! Keep in mind that we are covering only wheel brakes in this article, so we won't confuse you with brake master cylinders , parking brakes, or "Pascal's Law". There are only two different types of traditional braking systems used on all modern cars and light trucks: "drum brake systems" and "disc brake systems". (Your car may have both systems, one in the front, and the other in the rear.) While both set-ups use a friction material pressed against a rotating object to provide stopping power, they share almost no parts or part names. We've left off hybrid-electric braking systems as they use friction resistance from electrical generators to assist in the braking effort. First, disc brake systems.

These systems use a disc, also known as a rotor (same part, alternate name), which rotates along with your wheel/tire combo. To stop the rotation, brake pads , located within a caliper , press against the two sides of the disc or rotor. The caliper is the bridge between the hydraulic system and the wheel brakes, and it's the caliper which provides the force when the pads are clamped against the rotor. The resulting friction slows down the disc, and the vehicle also slows down. Disc brakes are very efficient. Their design allows for lots of air to circulate around the rotor, which carries away the tremendous heat generated from the friction. The pad location within the caliper makes pad replacement a rather straightforward service job on most vehicles. All light duty vehicles sold in the U.S. in the past few decades have used disc brakes in the front; many, but not all, modern cars and trucks also have disc brakes in the rear. Drum brake systems, which pre-date disc brakes, use different wheel brake components. There is no name sharing between the two systems!

The rotating member here is the brake drum ; the drum mounts over a pair of brake shoes , and the shoes, which contain the friction surface and are secured to a backing plate, sit inside the drum. A set of drum brake shoes. A set of disc brake pads by EBC Brakes . A disc brake caliper by Wilwood . A brake drum used in a typical automotive drum brake system. Big Disc Brakes for Trucks - How Truck Brakes Work | HowStuffWorks Big Disc Brakes for Trucks Prev Next Some trucks have disc brakes instead of S-Cam brakes. Air pressure works on the brake chamber and the slack adjuster in the same way that it does in S-Cam brakes. However, a power screw replaces the S-Cam. The power screw is turned by t­he pressure on the brake chamber and the slack adjuster. Then the power screw grasps the disc or rotor between a caliper's brake lining pads. The many benefits of air disc brakes include the following: Up Next Compared to drum brakes, air disc brakes reduce stopping distances by almost 40 percent. Because pressure is continually applied, it's easier for the vehicle to come to a complete stop. Air disc brakes almost completely stop brake fade.

Air disc brakes don't increase the wear of brake linings on a trailer or disc pads on a tractor. Still, despite all of the advantages, change isn't always embraced by the masses. In the United States, no regulation demands the use of air disc brakes, which are viewed as a high-end item because they're about twice the cost of drum brakes. Currently, about 95 percent of the U.S. heavy-duty truck market relies on drum brakes. In Europe, more than 80 percent of commercial trucks already use air disc brakes. The complete shift to air disc brakes may require 10 to 15 years. They say that a picture is worth a thousand words. Let's cruise to the next page to get a glimpse of a diagram of a truck's braking system. ­ Testing, Testing The National Highway Traffic Safety Administration (NHTSA) of the U.S. Department of Transportation believes that using air brakes will result in fewer collisions in emergency situations than using drum brakes. The NHTSA states that reducing the minimum stopping distance required by 30 percent would result in a significant number of lives saved. These conclusions were formed following simulator tests conducted at the University of Iowa [source: National Transportation Safety Board ].

Our Products, Pneumatic Clutches Manufacturer from New Delhi. Pneumatic Drum Clutches We offer our clients Pneumatic Drum Clutches Brakes (TAT / CB Type) as per the customer’s requirements. Working Principle :- The Pneuflex units are used as both clutches and brakes in general power transmission applications on all types of machinery. The primary feature of this design is the pneuflex torsional resilient principle made possible by the transmission of the power through sidewalls of the flexible rubber and cord actuating tube. These expand radially inwards when pressurized. This constricting tube forces friction shoes against cylindrical drums. More is the pressure more is the torque transmitted. The flexibility also permits minor shaft misalignments. Hydrocyclone Desilter Hydrocyclones are simple mechanical devices, without moving parts, designed to speed up the settling process of solids. Drilling fluid containing solids is fed tangentially into the body of the hydrocyclone, where centrifugal force induced by the flow causes heavier solids to separate from lighter solids. The heaviest solids are discharged out the bottom (underflow) of the hydrocyclone and the lightest solids exit the top (overflow).

The separation of solids is dependent upon feed particle size, fluid viscosity, fluid feed pressure (as a function of fluid density) and hydrocyclone size. Reliable Industrial Corporation offers two (2) models of hydrocyclones: desanders (12-inch cones) and desilters (4-inch cones). Fore each application, RELIABLE will determine the cone configuration and the pump and motor requirements necessary to maximize liquid/solids separation. Combination Clutch Brake Offfering Combination Clutches Brakes PCB Type The PNEUTORQ Clutch / Brake combination unit consists of an air actuated disc clutch and spring applied Disc Brake. Its compact design combined with its high thermal capacity and low air consumption makes it ideal for high speed, high cyclic, heavy-duty industrial applications. When cylinder is pressurized, the piston clamps the clutch disc to the hub, which transmits the torque to shaft. When cylinder exhausts, the springs moves the piston in the opposite direction clamping the brake disc & stopping the shaft. Overlap between Clutch and Brake is completely eliminated.

Pneutorq offers a wide range of Pneumatic Clutch/Brake units for use in Power presses, Shearing machines, Wood working machines & Special Purpose machines with the large range of sizes and capacities of Clutch Torque 260 NM to 26000 NM and Brake Torque from 180 NM to 18000 NM are available in numerous models. Brake Band Shoes For Oil Drilling Rigs The lining is woven in one piece on massive looms, under proper tension to insure a tightly woven superior quality product. Impregnated by vacuum process with high heat resisting compound, cured under tremendous heat and pressure, and specially treated, Lining is impervious to water, oil and grease. Abrasive-free fiber, wire insertion with infusible resins, tight weave, and pressure curing eliminate flange scoring tenden-cies and assure longer life. Brake Blocks are formed to the precise arc of the flanges, are easy to install, and can be counted on to conform to the flanges during a long life of dependable performance. They are recommended for deepest well digging under all conditions.

New Drawworks – Our engineering department keeps abreast of the latest engineering developments in draw-works. We are in a position to furnish the proper size brake blocks for newest type drilling rigs. Brake Blocks are drilled to fit each individual draw works and are also available undrilled. Desander for Oil Field Equipment Hydrocyclones are simple mechanical devices, without moving parts, designed to speed up the settling process of solids. Drilling fluid containing solids is fed tangentially into the body of the hydrocyclone, where centrifugal force induced by the flow causes heavier solids to separate from lighter solids. The heaviest solids are discharged out the bottom (underflow) of the hydrocyclone and the lightest solids exit the top (overflow). The separation of solids is dependent upon feed particle size, fluid viscosity, fluid feed pressure (as a function of fluid density) and hydrocyclone size. Reliable Industrial Corporation offers two (2) models of hydrocyclones: desanders (12-inch cones) and desilters (4-inch cones). Fore each application, RELIABLE will determine the cone configuration and the pump and motor requirements necessary to maximize liquid/solids separation.

How the braking system works | How a Car Works How the braking system works drum Dual-circuit braking system A typical dual-circuit braking system in which each circuit acts on both front wheels and one rear wheel. Pressing the brake pedal forces fluid out of the master cylinder along the brake pipes to the slave cylinders at the wheels; the master cylinder has a reservoir that keeps it full. Modern cars have brakes on all four wheels, operated by a hydraulic system . The brakes may be disc type or drum type. The front brakes play a greater part in stopping the car than the rear ones, because braking throws the car weight forward on to the front wheels. Many cars therefore have disc brakes , which are generally more efficient, at the front and drum brakes at the rear. All-disc braking systems are used on some expensive or high-performance cars, and all-drum systems on some older or smaller cars. Brake hydraulics Master and slave cylinders The master cylinder transmits hydraulic pressure to the slave cylinder when the pedal is pressed. When you push the brake pedal it depresses a piston in the master cylinder , forcing fluid along the pipe. The fluid travels to slave cylinders at each wheel and fills them, forcing pistons out to apply the brakes.

Fluid pressure distributes itself evenly around the system. The combined surface 'pushing' area of all the slave pistons is much greater than that of the piston in the master cylinder. Consequently, the master piston has to travel several inches to move the slave pistons the fraction of an inch it takes to apply the brakes. This arrangement allows great force to be exerted by the brakes, in the same way that a long-handled lever can easily lift a heavy object a short distance. Most modern cars are fitted with twin hydraulic circuits, with two master cylinders in tandem, in case one should fail. Sometimes one circuit works the front brakes and one the rear brakes; or each circuit works both front brakes and one of the rear brakes; or one circuit works all four brakes and the other the front ones only. Under heavy braking, so much weight may come off the rear wheels that they lock, possibly causing a dangerous skid. For this reason, the rear brakes are deliberately made less powerful than the front. Most cars now also have a load-sensitive pressure-limiting valve . It closes when heavy braking raises hydraulic pressure to a level that might cause the rear brakes to lock, and prevents any further movement of fluid to them.

Advanced cars may even have complex anti-lock systems that sense in various ways how the car is decelerating and whether any wheels are locking. Such systems apply and release the brakes in rapid succession to stop them locking. Power-assisted brakes Many cars also have power assistance to reduce the effort needed to apply the brakes. Usually the source of power is the pressure difference between the partial vacuum in the inlet manifold and the outside air. The servo unit that provides the assistance has a pipe connection to the inlet manifold. Diaphragm A direct-acting servo is fitted between the brake pedal and the master cylinder. The pedal can work the master cylinder directly if the servo fails or if the engine is not running. A direct-acting servo is fitted between the brake pedal and the master cylinder. The brake pedal pushes a rod that in turn pushes the master-cylinder piston. But the brake pedal also works on a set of air valves, and there is a large rubber diaphragm connected to the master-cylinder piston. When the brakes are off, both sides of the diaphragm are exposed to the vacuum from the manifold. Pressing the brake pedal closes the valve linking the rear side of the diaphragm to the manifold, and opens a valve that lets in air from outside.

The higher pressure of the outside air forces the diaphragm forward to push on the master-cylinder piston, and thereby assists the braking effort. If the pedal is then held, and pressed no further, the air valve admits no more air from outside, so the pressure on the brakes remains the same. When the pedal is released, the space behind the diaphragm is reopened to the manifold, so the pressure drops and the diaphragm falls back. If the vacuum fails because the engine stops, for example the brakes still work because there is a normal mechanical link between the pedal and the master cylinder. But much more force must be exerted on the brake pedal to apply them. How the brake servo works Brake off - both sides of the diaphragm are under vacuum. Applying the brake lets air in behind the diaphragm, forcing it against the cylinder. Some cars have an indirect-acting servo fitted in the hydraulic lines between the master cylinder and the brakes. Such a unit can be mounted anywhere in the engine compartment instead of having to be directly in front of the pedal. It, too, relies on manifold vacuum to provide the boost.

Pressing the brake pedal causes hydraulic pressure build up from the master cylinder, a valve opens and that triggers the vacuum servo. Disc brakes hose Disc brake The basic type of disc brake, with a single pair of pistons. There may be more than one pair, or a single piston operating both pads, like a scissor mechanism, through different types of calipers - a swinging or a sliding caliper. A disc brake has a disc that turns with the wheel. The disc is straddled by a caliper , in which there are small hydraulic pistons worked by pressure from the master cylinder. The pistons press on friction pads that clamp against the disc from each side to slow or stop it. The pads are shaped to cover a broad sector of the disc. There may be more than a single pair of pistons, especially in dual-circuit brakes. The pistons move only a tiny distance to apply the brakes, and the pads barely clear the disc when the brakes are released. They have no return springs . When the brake is applied, fluid pressure forces the pads against the disc. With the brake off, both pads barely clear the disc.

Rubber sealing rings round the pistons are designed to let the pistons slip forward gradually as the pads wear down, so that the tiny gap remains constant and the brakes do not need adjustment. Many later cars have wear sensors leads embedded in the pads. When the pads are nearly worn out, the leads are exposed and short-circuited by the metal disc, illuminating a warning light on the instrument panel. Drum brakes springs Drum brake A drum brake with a leading and a trailing shoe, which has only one hydraulic cylinder; brakes with two leading shoes have a cylinder for each shoe and are fitted to the front wheels on an all-drum system. A drum brake has a hollow drum that turns with the wheel. Its open back is covered by a stationary backplate on which there are two curved shoes carrying friction linings. The shoes are forced outwards by hydraulic pressure moving pistons in the brake's wheel cylinders , so pressing the linings against the inside of the drum to slow or stop it. With the brakes on, the shoes are forced against the drums by their piston. Each brake shoe has a pivot at one end and a piston at the other. A leading shoe has the piston at the leading edge relative to the direction in which the drum turns.

The rotation of the drum tends to pull the leading shoe firmly against it when it makes contact, improving the braking effect. Some drums have twin leading shoes, each with its own hydraulic cylinder; others have one leading and one trailing shoe - with the pivot at the front. This design allows the two shoes to be forced apart from each other by a single cylinder with a piston in each end. It is simpler but less powerful than the two-leading-shoe system, and is usually restricted to rear brakes. In either type, return springs pull the shoes back a short way when the brakes are released. Shoe travel is kept as short as possible by an adjuster. Older systems have manual adjusters that need to be turned from time to time as the friction linings wear. Later brakes have automatic adjustment by means of a ratchet. Drum brakes may fade if they are applied repeatedly within a short time - they heat up and lose their efficiency until they cool down again. Discs, with their more open construction, are much less prone to fading.

The handbrake Lever The handbrake mechanism The handbrake acts on the shoes by means of a mechanical system, separate from the hydraulic cylinder, consisting of a lever and arm in the brake drum; they are operated by a cable from the handbrake lever inside the car. Apart from the hydraulic braking system, all cars have a mechanical handbrake acting on two wheels - usually the rear ones. The handbrake gives limited braking if the hydraulic system fails completely, but its main purpose is as a parking brake . The handbrake lever pulls a cable or pair of cables linked to the brakes by a set of smaller levers, pulleys and guides whose details vary greatly from car to car. A ratchet on the handbrake lever keeps the brake on once it is applied. A push button disengages the ratchet and frees the lever. On drum brakes, the handbrake system presses the brake linings against the drums. Disc brakes sometimes have a comparable handbrake arrangement, but because it is difficult to place the linkage on a compact caliper, there may be a completely separate set of handbrake pads for each disc.

The ultimate video course We take this car to pieces and then build it again, explaining how every single part works. By the time you finish watching this, you'll understand everything inside a car. Watch us take a Mazda MX5 Miatta to pieces, and then build it back together again into a modern working car. Every part explained in detail. We've created the most detailed 3D model ever produced so we can show you everything working. Types of Brakes - Levin Tire & Service Center Levin Tire & Service Center Tire & Service Network Types of Brakes Brakes are one of the most important safety features on your vehicle. There are different types of brakes, both between vehicles and within a vehicle. The brakes used to stop a vehicle while driving are known as the service brakes, which are either a disc and drum brake. Vehicles also come equipped with other braking systems, including anti-lock and emergency brakes. Disc Brakes Disc brakes consist of a disc brake rotor - which is attached to the wheel - and a caliper, which holds the disc brake pads. Hydraulic pressure from the master cylinder causes the caliper piston to clamp the disc brake rotor between the disc brake pads.

This creates friction between the pads and rotor, causing your car to slow down or stop. Drum Brakes Drum brakes consist of a brake drum attached to the wheel, a wheel cylinder, brake shoes, and brake return springs. Hydraulic pressure from the master cylinder causes the wheel cylinder to press the brake shoes against the brake drum. This creates friction between the shoes and drum to slow or stop your car. Emergency Brakes Vehicles also come equipped with a secondary braking system, known as emergency, or parking brakes. Emergency brakes are independent of the service brakes, and are not powered by hydraulics. Parking brakes use cables to mechanically apply the brakes (usually the rear brake). There are a few different types of emergency brakes, which include: a stick lever located between the driver and passenger seats; a pedal located to the left of the floor pedals; or a push button or handle located somewhere near the steering column. Emergency brakes are most often used as a parking brake to help keep a vehicle stationary while parked. And, yes, they are also used in emergency situations, in case the other brake system fails! Anti-Lock Brakes Computer-controlled anti-lock braking systems (ABS) is an important safety feature which is equipped on most newer vehicles.

When brakes are applied suddenly, ABS prevents the wheels from locking up and the tires from skidding. The system monitors the speed of each wheel and automatically pulses the brake pressure on and off rapidly on any wheels where skidding is detected. This is beneficial for driving on wet and slippery roads. ABS works with the service brakes to decrease stopping distance and increase control and stability of the vehicle during hard braking. Related Articles Bleeding the brakes | How a Car Works The front and rear brakes of a dual-brake system have separate hydraulic circuits. The configuration can vary depending on the manufacturer. The brake pipes also pass through a balance unit which limits pressure to the rear brakes to prevent the back wheels locking when the weight of the car is thrown forwards during braking. On some cars, the rear brakes cannot be bled if the weight is taken off the wheels, and the job must be carried out with the car on the ground. The fluid is hygroscopic , which means that over a period of time it absorbs water from the atmosphere, and its boiling point is lowered. Friction caused by continual heavy braking heats the fluid in disc calipers or wheel cylinders , and its water content turns to steam.

The brake pedal feels 'spongy' when you press it, and in severe cases the brakes will fail completely. Air leaking into the hydraulic system makes the pedals spongy because, unlike brake fluid, it can be compressed. If the pedal feels spongy before the fluid is due for replacement, check the system for leaks. Air can be drawn in through leaking seals or faulty brake pipes (See Looking for leaks in the brake system ). Disc brake Locating bleed nipples If you find a leak, first replace the faulty components and then renew the brake fluid by bleeding the complete hydraulic system. The fluid is drained from the system by opening bleed nipples - which are small valves - on each caliper or wheel cylinder in turn and pumping the brake pedal. At the same time the master-cylinder reservoir has to be topped up with new fluid at frequent intervals, because its level drops as the old fluid is drained off. If the reservoir empties, air will get into the system and the whole process will have to be repeated until all air is expelled. Keep it topped up as near as possible to the level mark on the side - you need a helper, unless you have a bleed pipe with a non-return valve in it (See Bleeding the clutch ).

Avoid spilling brake fluid on paint-work - it will start to penetrate within a few seconds. If any splashes on, hose it off immediately and wipe the area with a clean rag. Wash your hands thoroughly if it gets on your fingers. On many cars you can reach the bleed nipple with the road wheel in place. But it is usually simpler to remove the wheel (See Checking, removing and refitting road wheels ). On dual-circuit brake systems, it is usual to start bleeding at the front-wheel brake which is nearest to the master cylinder - normally the one on the driver's side. Bleed the other front wheel and then the rear wheels, ending with the one furthest from the master cylinder. Cars with servo-assisted brakes (See Replacing a master cylinder and servo unit ) may have a bleed nipple on the servo unit. If so, start with that. Four-piston system bleed sequence There are two sequences for bleeding a four-piston system, involving one nipple at the rear and two at the front. A variation of the dual-circuit system has front disc-brake calipers with two pistons and cylinders in each caliper. Each cylinder has a bleed nipple, so that there are at least two nipples on each caliper.

One half of the master cylinder feeds fluid to the back brakes and to one of the pistons in each caliper; the second half feeds the other two pistons. A typical bleeding sequence is to start with a rear brake then bleed the two front-caliper nipples on the same side of the car. Repeat the sequence on the other side. There are two sequences for bleeding a four-piston system, involving one nipple at the rear and two at the front . Removing air from a brake system The bleeding method for drum brakes , disc brakes and combination systems is the same. Different brake types on the same car may have different nipple sizes, and you must use the correct size of ring spanner for each one. If a road wheel has to be removed, support the car on axle stands and chock the other wheels. Clean off dirt and corrosion around nipple with a wire brush. Wipe the nipple clean with a piece of rag. Remove dirt and corrosion from the bleed nipple with a wire brush , take off the dust cap (if there is one) and put a ring spanner of exactly the right size over the nipple. Leave the spanner in place and attach a length of clear plastic tubing to the nipple.

The tubing should be about 2 ft (600 mm) long, and an airtight fit on the nipple. Place a clean glass container, such as a jam jar, under the nipple, and put the free end of the tube into it. Pour enough fresh brake fluid into the jar to cover the end of the tube. Take care when opening the bleed nipple. They are often difficult to turn, and fragile enough to fracture if excessive force is used. The nipple is made of soft metal and can be damaged easily if the spanner is a little too large. A corroded nipple can easily break off; if it does, the remains may come out with a screw extractor . Otherwise you may have to buy a new brake caliper or wheel cylinder. If the nipple does not turn with a reasonable amount of force, put a drop of penetrating oil on the thread and allow a few minutes for the oil to penetrate. Loosen the bleed nipple about half a turn, and leave the spanner in place. Brake fluid should begin to ooze from the nipple and flow down the tube into the jar. If it does not, the nipple may be blocked, and you must open it (see top right).

When the brake fluid is flowing, ask your helper to pump the brake pedal while you watch the fluid run through the clear tubing. More than a few strokes may empty the master-cylinder reservoir, so remember to keep checking it and top it up if necessary. After bleeding, be sure to retighten the nipple before removing the bleed tube. If you see bubbles - they may be quite small, like froth - there is air in the system. Continue pumping (and topping up) until there are no more. Tell your helper to pump twice more, then keep the pedal depressed. Close the nipple by tightening it with the spanner. Remove the tubing and the spanner, replace the road wheel and move to the next nipple. Fit the correct size of ring spanner over the nipple, then attach the bleed tube. Loosen the nipple about half a turn - fluid should flow into the jar. Make sure that the end of the tube is below the fluid in the jar. Bleeding brakes using a tube with a non-return valve (See Bleeding the clutch ) involves exactly the same procedure, but you do not need a helper. Frequent checks must be made to see if air bubbles are passing through the tube.

The same methods are used if you are bleeding the system simply to replace fluid, and not expel air. Six strokes of the brake pedal with each nipple open in turn should pump out the old fluid and replace it with new in each part of the system. Make sure you retighten each nipple before moving to the next. The pumping sequence may vary with different types of master cylinder. When you have bled all the nipples, apply hard foot pressure to the brake pedal. It should no longer feel spongy. If it does, there is still air in the system. Repeat the bleeding process, possibly in a different sequence, until the pedal feels firm. It should remain equally firm under hard and prolonged pressure, without sinking to the floor. To test the brakes on the road, wait until there is no traffic, drive slowly for a short distance and press the brake pedal as you would normally. The car should pull up quickly and in a straight line, and the pedal should not feel spongy. Drive on at normal speed and apply the brakes firmly. Again, the car should stop quickly, in a straight line and in about the same distance. Try a few more applications - if the sponginess returns, there is still air in the system.

Repeat the bleeding process until you are satisfied that the brakes are operating correctly. If your bleed nipples are not fitted with dust caps, buy some and fit them after bleeding. Opening blocked nipples Brake-hose clamp If bleeding does not begin when you open a nipple, it is probably blocked. Use a brake-hose clamp (See Looking for leaks in the brake system ) to seal the flexible hose leading to that brake, take off the tubing and unscrew the bleed nipple completely. Carefully poke the nipple clear with a thin wire. Carefully poke the nipple clear with a piece of stiff, thin wire. Replace the nipple tightly, refix the flexible tubing to it and remove the hose clamp . Do not attempt to clear a blocked nipple without removing it; dirt dislodged could find its way into the slave cylinder . Loosen the bleed nipple half a turn, and the fluid should begin to flow. Pumping correctly for different master cylinders An aluminium-bodied centre-valve (CV) master cylinder (single-circuit type). There are two types of master cylinder - the aluminium-bodied centre-valve (CV) type and the cast-iron compression-barrel (CB) type. Each calls for a different pump action in the brake pedal when you bleed it.

That applies whether they are single or dual-system units. With the CV type, push the pedal down fully, then give it three short, quick strokes near the bottom of its travel and release it fully. A cast-iron compression-barrel (CB) master cylinder (dual-circuit type) Repeat at once, and continue the sequence until there is no more air coming from the bleed nipple. With the CB type, push the pedal down fully to the floor, then let it rise slowly. Wait for three or four seconds, then repeat the process until all air has been expelled. If one method of pumping does not clear bubbles from the fluid, and you have tried a different bleed sequence, use the other pumping action. The ultimate video course We take this car to pieces and then build it again, explaining how every single part works. By the time you finish watching this, you'll understand everything inside a car. Watch us take a Mazda MX5 Miatta to pieces, and then build it back together again into a modern working car. Every part explained in detail. We've created the most detailed 3D model ever produced so we can show you everything working. Brake Trends: Drums vs.

Discs - Article - TruckingInfo.com Brake Trends: Drums vs. Discs Although 90% of heavy trucks still get drums, discs are gaining in popularity. July 2014, TruckingInfo.com - Feature SHARING TOOLS | Print Subscribe Five years ago, when federal authorities announced that stopping-distance rules for heavy tractors would get stricter, there was talk that this would finally cause a conversion from drum brakes to air disc brakes in the U.S. Even well before that, some supplier representatives claimed that discs were so superior that a switch to them was inevitable, just as has happened in Europe and other overseas markets. Yet here we are, a year after the final rules change became effective and more than three decades since they first appeared in North America, and disc brakes still comprise a small minority of air brakes being bought by truck operators. Put another way, 90% of heavy truck buyers today still choose tried-and-true drum brakes, according to manufacturers we talked with. These are not good ol’ drum brakes, but improved designs that stop better and last longer than past products. They have to perform better to meet Federal Motor Vehicle Safety Standard 121’s shorter limits, and to do so many of them are larger.

That, in turn, allows their linings to stay in service for more miles, supplier reps say. ”Air disc brakes cost more, so there isn’t a more cost-effective way to stop a truck than a drum brake,” says Joe Kay, director of engineering for brake systems at Meritor, which offers both types. He means on tractors, which the “121” rules affected, and for which drum brakes were specifically improved. But discs can stop those same tractors quicker, which is why some fleets are adopting them. And discs hold up better in severe-service applications, like trash collection trucks, many of whose operators have embraced them. “There’s an added expense with disc brakes and operators need time to get a proper payback,” says Gary Ganaway, director of marketing and global customer solutions at Bendix Spicer Foundation Brake, which also offers both drums and discs. Cost for air disc brakes varies by truck builder: At Kenworth and Peterbilt they’re standard on the steer axle. For all the other builders the cost of upgrading is $800 to $1,000 per axle. In some cases there can be a payback through maintenance. With discs, “refuse trucks doubled their brake life,” Ganaway says.

With drums, “they get hot and wear rates increase exponentially.” It’s easier and quicker to change a set of pads on disc brakes than shoes on drums, Kay and Ganaway both note. Pads take as little as 15 minutes to change out compared to almost an hour for a set of shoes. That’s where the maintenance payback can come into play. But the advantage is less if original linings on a drum-brake system can be made to last a long time, which is possible with larger brakes. To meet 121 stopping requirements, steer axle brakes have gone from 15x4 inches to 16½x5 inches, while drive-axle brakes stayed with 16½x7s, Ganaway explains. Daimler Trucks chose to make 16½x8-5/8-inch drum brakes standard on drive axles for Freightliners and Western Stars. However, there is a movement to air disc brakes and it’s accelerating.

“We announced the sale of our half a millionth air disc brake at the Louisville truck show, which was only two years after we announced the first 250,000th” following introduction of current models in 2005, says Bendix Spicer’s Ganaway. “Fleets are paying more attention to foundation brakes and began experimenting with air disc brakes. They are becoming more and more comfortable and are coming back for more.” “Air disc brakes are progressively increasing,” says Steve Hampson, director of sales and marketing at Wabco, which sells discs. “There are two levels of fleets: those looking for brake performance — like tankers and heavy steel haulers — and those who want a longer lifetime. With discs you don’t have to touch the brakes as often.” Adoption percentages vary: On trailers it’s 8 to 10%, on trucks and tractors it’s 12 to 15%. The usual configuration for tractors is discs front and drums rear. Brake adjustment is another reason for disc brake growth. Out-of-adjustment brakes are often an issue in roadside inspections, and citations now affect a fleet’s Compliance, Safety and Accountability ratings, Hampson adds.

“Disc brakes are encapsulated and have fewer moving parts, so out-of-stroke is not an issue. Some fleets are looking at disc brakes so they don’t have the concern over adjustment.” Disc brakes are inherently self-adjusting and have no “stroke” to measure, so are difficult for inspectors to gauge unless they have dynamometers to measure performance. Some states do. Aside from refuse fleets, firms adopting discs are those sensitive to safety, like haulers of petroleum, chemicals and industrial gases. Freight haulers who are squeezed more by costs are more likely to stick with drums. “Customers have to choose among disc brakes and electronic aids like lane-departure warnings, electronic stability control and so on, and they have only so many dollars to spend on safety,” says Meritor’s Kay. “There are also automated transmissions, for driver ease and fuel economy.” One who’s staying with drums is Kirk Altrichter, vice president, maintenance for Crete Carrier Corp. “Cost and weight” argue against discs, he says. He wasn’t clear on price, but puts the weight penalty at 100 or more pounds per axle. “We’re struggling constantly to take weight off,” he says.

“The cost for that is $1.50 per pound — that’s a figure we use. So there’s not enough justification to make discs worth it. Drums meet the stopping distance but the jury’s still out as far as how they’re lasting. I don’t think there’s extra life.” Air disc brakes might some day account for 25% of sales, says Ganaway. More bullish is Randy Petresh, vice president, technical services, at Haldex Commercial Vehicle Systems, which sells a variety of components for brakes and air systems. He thinks disc brake usage might go to 70% in 10 years. Even so, “There’s never going to be a complete conversion. There’s always going to be segments that will stick to drums no matter what. And cost — drums have been in production for 30, 40 years and tooling has been amortized, so drum-brake parts are dirt cheap.” Parts pricing is one reason air drum brakes are popular among buyers of medium-duty trucks. “In Class 6 and 7, greater than half the buyers buy air brakes,” says Meritor’s Kay.

“Some Class 5 buyers will spec air if they have other air systems on the truck, though the lower end of Class 5 use hydraulic discs. It depends on gross axle weight rating; there’s a point where hydraulic brakes just can’t do the job anymore, about 32,000 pounds.” Beefy drum brakes absorb heat and work better in many cases. “There are also parking concerns,” Kay continues. “Disc brake systems are more complex, and you don’t get the same power, so OEMs go with some kind of driveline brake.” Motorcoach Brake Systems and Safety Technologies | Federal Motor Carrier Safety Administration Federal Motor Carrier Safety Administration brake_safety_systems_02-14.pdf Overview This booklet provides information and guidance to motorcoach drivers who have the challenge of understanding and properly handling complex mechanical devices, technologies, and safety related systems. Each section of this booklet concludes with important summary points. Always refer to the manufacturer's operating instructions for specific vehicle model information. This booklet will answer the following questions: What is the "acceptable" air system leakage limit above which you should not operate the motorcoach? What does your air pressure gauge tell you? What does it mean when the ABS light is lit while you are driving?

When should cruise control not be used? Should you use a retarder on a slippery road? How much longer will it take you to stop when traveling at 60 mph with drum brakes instead of disc brakes after repeated brake applications? How much longer at 75 mph? Contents Air Brake Systems All air-braked motorcoaches built since March 1, 1975 for operation in the United States must meet minimum requirements known as "Federal Motor Vehicle Safety Standard 121, Air Brake Systems" or FMVSS 121. This booklet will cover the most common air brake system designs from 1975 to 2006. Air Compressor and Air System Pressure - You should be aware of common air pressure thresholds of your motorcoach's brake system. The air pressure gauge tells you the adequacy of the air system. Air compressors typically have a "cut-out" pressure between 110 psi and 130 psi and a "cut-in" pressure about 20 psi lower. Air pressure build-up from 85 psi to 100 psi should take 45 seconds or less. While driving, observe the air pressure gauge during build-up to ensure proper air system operation.

Every motorcoach is different, some by design and some by condition. Know what is "typical" for your motorcoach. To test air system integrity, make and hold a full brake application for at least 2 minutes while watching the air pressure gauge with theengine off, the key in the "on" position, and the park brake off. Leakage, if present, should be less than 5 psi for the 2 minutes. Continue the test by applying the park brake and then "pumping" the brake pedal to release air. A visual warning should activate when the air pressure gauge drops below 60 psi. Air pressure gauges - The most common air brake system designs since March 1, 1975 isolate the front axle system from the rear axle system and have two air pressure gauges. The gauges for both air systems show how much air is consumed during braking and when the compressor activates to build up pressure. If you notice frequent activation (compressor cut-in and cut-out) or excessive air loss in either system during braking, pull over at a safe place and perform the 2 minute / 5 psi leakage test.

If a failure occurs in either the front or rear axle system, there is usually enough air in the other axle system to bring the vehicle to a safe stop. You should seek brake system service immediately if one system fails. Caution about park brakes - Remember the park brakes are for parking only and are not designed as emergency brakes. Always leave the transmission in neutral (both manual and automatic) when applying the park brakes. Important Points about Air Brake Systems Air Leakage - Frequent compressor cycling usually indicates an air loss somewhere in the air brake system. It is unsafe to operate a motorcoach with a noticeable air leak. Even though frequent compressor cycling may maintain adequate air system pressure, you should seek a brake system inspection without delay. Air Pressure Gauges - It is advisable to continuously monitor the gauges for early detection of system malfunction such as unusual or imbalanced air consumption. Any leak in the air system can affect the braking performance of your motorcoach. Low Pressure Warning - In most cases, you should notice an air leak or malfunction before getting a low pressure warning. When a low pressure warning occurs, immediately bring the motorcoach to a safe stop, off of the roadway.

Continuing to operate the motorcoach could result in an automatic application of the park brakes, possibly leading to a loss of control or a stop in an unsafe position. Continual Awareness - Know your motorcoach, its features, and its condition at start-up and throughout the trip. Proper Use of Park Brakes - The park brakes should only be used to park. Do not use the park brakes to stop the motorcoach during an emergency. Be prepared to handle a malfunction at any time. Anti-Lock Braking Systems (ABS) Since March 1, 1998, new motorcoaches are required to have an anti-lock braking system (ABS). An ABS monitors all wheels for skidding during brake application and momentarily releases some or all of the brake action until the skid stops. The basic function of an ABS is to prevent wheel lock and provide directional control during braking. ABS keeps the wheels rolling for optimum braking and steering. On some slippery surfaces, ABS can provide wheel lock control that may not be readily detected by the driver. Because most highway travel is on dry pavement and the typical brake application is less than 20 psi, the ABS seldom activates.

It is important for you to know whether the motorcoach that you are operating has an ABS. If your motorcoach is ABS equipped, you should know how to use this technology and be familiar with the signals of ABS malfunction. ABS Lamp on the Instrument Panel - Turn the ignition key to "on" and look for the yellow lamp, light, or lighted icon on the instrument panel marked ABS. After engine start-up, the system does a self-diagnosis and, if no malfunction is found, the lamp will turn off after a few seconds. When the motorcoach speed approaches 6 to 10 mph, the system checks for proper wheel speed signals. [Note: On some earlier production systems, the lamp remains lit until the motorcoach speed reaches 6 to 10 mph.] The sequence at motorcoach start-up is: Lamp On → System Check → Followed by Lamp Off → System OK Critical malfunctions or signals that shut down all or part of the system will activate the ABS lamp.

The two most important messages to remember are: Vehicle Start-up → No Yellow ABS Light → Means No ABS While Driving →Yellow ABS Light Illuminates → Means ABS Malfunction What to do when the ABS light illuminates - If the yellow light illuminates while driving, the ABS is malfunctioning and should be inspected as soon as possible. The malfunction could be as simple as a loose wire or sticking modulator valve. It is a safety violation to operate a motorcoach with a malfunctioning ABS. ABS malfunctions leave the motorcoach with a traditional braking system which means the driver has more responsibility for motorcoach braking and handling. What to do when ABS activates - When ABS activates, you may or may not feel the ABS operating. When you must slow or stop the motorcoach quickly and whenever ABS activates, keep a steady brake application and let the ABS modulate the brake pressure as needed. You will not see the yellow ABS light when ABS activates. The purpose of the ABS light is to tell you the ABS may be malfunctioning and needs inspection. ABS Based Safety Technologies ABS based technologies include traction control systems, electronic stability control systems, and anti-roll systems. These systems, through the use of additional sensors, will apply the motorcoach's brakes under certain circumstances.

Traction Control Systems use a sensor to monitor and control traction loss when increased engine output or roadway conditions cause wheel slippage. The system reduces power to the slipping wheel(s) until traction is regained. For optimum control and safety, traction control systems keep the wheels rolling at the same speed. A motorcoach equipped with a traction control system may have an ON-OFF switch and a green light marked TC, ATC, or Traction Control. The preferred operating mode is to leave the traction control system switch in the "ON" position. When driving in mud, snow, or ice, and one drive wheel starts to spin, the computer tells the traction control valve to apply the brake at the spinning wheel and transfer the engine power to the opposite wheel. This function, known as differential braking, may also tell the engine electronic control unit to reduce engine power. While climbing a hill on wet pavement, you cross a bridge and discover the surface is icy and the drive wheels start to spin. You must immediately back off the throttle to maintain steering and control. A Typical Scenario With Traction Control While climbing a hill on wet pavement, you cross a bridge and discover the surface is icy and the drive wheels start to spin.

The traction control system senses the wheel slippage and automatically reduces power to the slipping wheels until traction is regained. Electronic stability control systems are totally automatic and use roll, pitch and yaw sensors to determine whether the motorcoach is leaning, changing direction, or slowing down. Such systems typically look for lane changes and similar maneuvers where motorcoach speed is too fast for conditions and countermeasures are advisable. A typical maneuver could call for the traction control valve and the brake modulator valve to apply a small amount of braking to one or more wheels to assist the turn and maintain vehicle control. Some systems also use a steering wheel position sensor. Anti-roll systems are also totally automatic. They use a sophisticated set of algorithms to interpret steering wheel position, yaw and roll, and other signals to predict potential rollover and apply the brakes to help prevent a rollover. Know whether your motorcoach is equipped with an ABS or traction control system, and whether the ABS is functioning and the traction control system is turned on. Slippery road -- motorcoach has ABS -- start to brake -- ABS starts cycling -- what should you do? Keep your foot on the brake do not try to pump or modulate the brake.

Bring the motorcoach to a stop or a safe speed before releasing the foot brake. Remember that ABS always beats "driver best effort braking." Stopping distance is almost always longer with locked (skidding) wheels. Loss of directional control is another potentially serious consequence of wheel lock-up. ABS light on while driving means the ABS is malfunctioning. Be extra careful to avoid locking the wheels when braking. Learn all you can about ABS, traction control systems, and other safety technologies even if the motorcoach you currently drive is not equipped with such technologies. For safety concerns, some motorcoach companies prohibit their drivers from using cruise control. Cruise control should be used with caution and sparingly. It should not be used under the following circumstances: During adverse driving conditions such as wet, icy, or slippery roads, During congested highway conditions with heavy traffic, and During times when you are feeling fatigued. Because of circadian rhythms in human alertness, increased sleepiness occurs during nighttime and the early afternoon. Cruise control should not be used during the dark of night. In addition, it should only be used with extreme caution, if at all, between 2 p.m. and 6 p.m.

To enhance safety, some motorcoaches are installed with an intelligent cruise control system. This system differs from a standard cruise control system by using forward looking sensors to monitor motorcoach closing speed and warn the driver of a potential rear-end collision with the vehicle ahead. Retarders Retarders give you another way to slow down, lessen the need to use the brakes, and reduce brake wear. All retarders can be turned on or off by the driver. As with cruise control, retarders should never be used on wet, icy, or slippery roads. We will discuss three types of retarders (exhaust brake, engine compression brake, and driveline retarder): (1) The exhaust brake uses a valve installed on the engine exhaust system to restrict or hold back the engine exhaust. The closed valve causes a buildup of exhaust gases that flows back into the combustion chamber of the cylinders. Pressure buildup in the cylinders creates a power loss by restricting the fuel mixture delivery to the cylinders. (2) The engine compression brake (often referred to as a "Jake Brake") is installed in the engine cylinder head and directly controls the engine exhaust valves or throttle valves.

As a new driver, you learned that the compression of the engine alone will slow (retard) the motorcoach when you take your foot off the throttle. If an engine compression brake is installed on a 300 horsepower engine, you can expect up to 150 horsepower from the engine to slow the motorcoach. When the retarder is turned on, the engine compression brake activates when you remove your foot from the throttle. Most engine compression brakes have multiple settings. For example, the "Jake Brake" has three settings. The use of an engine compression brake is restricted or prohibited in some areas due to Federal, State, or local noise standards. (3) The driveline retarder works by generating force which causes the driveshaft to resist turning, thereby slowing the motorcoach. Hydraulic is the most common type of driveline retarder. The efficiency of driveline retarders (horsepower available to slow the motorcoach) is generally greater than exhaust brake and engine compression brake retarders. Driveline retarders can continuously provide smooth retardation over a wide range of speeds. Remember that activation only occurs when the retarder master switch is turned on.

It is important to remember that a hydraulic driveline retarder is cooled by the transmission oil and excessive use can cause overheating. The popularity of driveline retarders increased as more automatic transmission motorcoaches became equipped with electronic engine controls. For example, it is possible on newer motorcoaches to interconnect the retarder and cruise control to keep the motorcoach at a constant speed as it travels up and down hills. On other newer motorcoaches, the electronic control monitors throttle position, cruise control setting, motorcoach speed, and even transmission temperature, to recommend or select upshifts and downshifts. Warnings Do not use a retarder while driving on wet, icy, or slippery roads. Doing so can cause a loss of control. In cold weather, turn the retarder off when approaching bridge decks, on-ramps, or exit ramps. During slippery conditions, turn the retarder off using the master control (on/off) switch lever. If you experience a loss of directional control, immediately turn off the retarder. Brake Fade In the late 1990s, the majority of motorcoaches manufactured for operation in North America switched from being equipped with drum brakes to disc brakes because of better braking performance.

Disc brakes are less susceptible to "brake fade" or diminished braking capacity. Brake fade becomes an important factor when the brakes are applied while descending long grades. All drivers should know whether their motorcoach is equipped with drum or disc brakes. It is easy to look at the wheel area and determine the type of brakes on the motorcoach. A study comparing the braking effectiveness of air disc brakes versus air drum brakes for tractor-trailers was conducted by Bendix Commercial Vehicle Systems. Tested at 60 mph, the disc brake stopped the vehicle at about 200 feet and the drum brake did so at about 270 feet. This braking distance difference is the result of the different braking systems. After 15 stops from 60 mph, the stopping distance for the drum brake deteriorated from 270 feet to 450 feet, while stopping distance for the disc brake remained at close to the initial 200 feet. The increased braking distance for drum brakes at successive stops is the result of brake fade. The 75 mph results are even more dramatic. Both the early and later stops were both significantly affected by brake fade.

If disc versus drum brake testing was performed with motorcoaches at 60 mph and 75 mph, similar results would occur. Testing Results Beyond FMVSS 121 Fade Resistance 60 MPH - DISC vs. DRUM: Drum brake stopping distance increased from 270 ft. to 450 ft. with 15 brake applications. While disc brakes remained fairly constant at 200 ft. Testing Results Beyond FMVSS 121 Fade Resistance 75 MPH - DISC vs. DRUM: Drum brake stopping distance increased from 460 ft. to 780 ft. with 15 brake applications. While disc brakes remained fairly constant at 300 to 325 ft.

Question: Disc, drum, and air are all types of what?
Answer: Brakes