Patent Publication Number: US-2003234573-A1

Title: Brake system for a vehicle having an integral precharge pump and back-flow protection

Description:
[0001] This application claims the benefit of commonly assigned: U.S. Provisional patent application bearing the serial No. 60/389,973 titled ELECTRO-HYDRAULIC CONTROL UNIT WITH INTEGRAL PRE-CHARGE PUMP, by Reuter, et al; and US Provisional patent application bearing the serial No. 60/439,935, titled CONTROLLED BRAKE SYSTEMS FOR VEHICLES, by Reuter, et al. 
    
    
     
       TECHNICAL FIELD OF THE INVENTION  
       [0002] This invention relates to vehicle brakes, and more particularly to a vehicle with a brake apparatus including a pump providing brake fluid during controlled braking operation of the vehicle.  
       BACKGROUND OF THE INVENTION  
       [0003] Since the mid 1930s, vehicles such as automobiles and light trucks have predominantly utilized hydraulic brake systems having a pedal operated master cylinder supplying pressurized hydraulic fluid to disk or drum braking devices at each wheel.  
       [0004] Early hydraulic brake systems utilized a single hydraulic fluid circuit supplying pressurized fluid from the master cylinder to all four corners of the vehicle. A break in the fluid circuit anywhere rendered the entire hydraulic brake system inoperative.  
       [0005] In order to prevent a total loss of hydraulic braking in the event of a failure of part of the system, failsafe hydraulic split brake systems were developed that provided two separate fluid circuits from the master cylinder, configured such that a failure of either of the two fluid circuits would still leave hydraulic brakes operative on at least two corners of the vehicle. In rear wheel drive automobiles and light trucks, one fluid circuit typically served the front wheels, and the other fluid circuit served the rear wheels, to provide a front/rear (F/R) failsafe hydraulic split system. Front wheel drive vehicles typically used a diagonal failsafe hydraulic split system, having one front corner and the diagonally opposite rear corner of the vehicle on one fluid circuit, and the other front corner and its diagonally opposite rear corner on the second fluid circuit. These failsafe provisions were incorporated into government regulations that required brake systems to be configured such that a single failure of the braking system would still leave the brakes on at least two corners of the vehicle operational.  
       [0006] In the years since hydraulic brake systems became the norm, many additional features have been added to further enhance operation and optimize vehicle performance. Modern brake systems often include a booster that amplifies force exerted on the brake pedal, to provide power brakes that allow a person operating the vehicle to control the brakes with significantly less force on the brake pedal than is required in a non-boosted brake system. Anti-lock brake systems (ABS) were developed in which valves controlling fluid flow to each corner of the vehicle. were pulsed, in response to signals received from rotation sensors monitoring each wheel, to preclude locking the brakes on slippery road surfaces. Traction control systems (TCS) were added that controlled both the brakes and the engine throttle setting to improve traction and handling of the vehicle during maneuvers, such as acceleration or turning, when the brakes are not being applied by the operator. Vehicle dynamics control (VDC) further advanced the level of sophistication of brake systems to utilize a number of sensors throughout the vehicle, and a more advanced onboard computer with higher throughput, to monitor forces acting on the vehicle, together with inputs indicating operational commands from the operator applied to the steering, braking, and drive systems. VDC analyzes the data received from the sensors and coordinates operation of the various elements of the vehicle brake system, power-train, and suspension to provide enhanced vehicle safety or performance of the vehicle.  
       [0007] The addition of all of these enhancements has made hydraulic brake systems very complex. Numerous valves, sensors, and electronic control components are required. Brake systems offering one or more types of automated control operating modes, such as ABS, TCS and VDC, are known as “controlled braking systems.” 
       [0008] Recent advances in technology have made it feasible to develop a controlled brake system that utilizes electrically actuated brakes, rather than hydraulic brakes, on at least the rear corners of a vehicle. Such brake systems are known as “Hybrid” brake systems. Commonly assigned U.S. patent application Ser. No. 10/121,454, titled Hybrid Brake System for a Vehicle, by Reuter, et al, describes such a system, and is incorporated herein by reference.  
       [0009] To provide a flow of pressurized brake fluid during controlled braking operation in modes such as TCS and VDC where there is no brake pressure being produced by the master cylinder because the brake pedal is not depressed, controlled braking systems typically include a controlled braking pump driven by an electric motor for re-circulating pressurized brake fluid through the controlled braking circuit during one or more of the controlled braking operations. Such controlled braking pumps must also be capable of starting and producing braking pressure in a fraction of a second when the braking system recognizes the need for and initiates a controlled braking operation.  
       [0010] A controlled braking pump must further be an efficient suction-type pump, since fluid must be drawn from the remote master cylinder reservoir and subsequently pumped through extended brake lines to the wheel brake. At low ambient operating temperatures, the brake fluid becomes more viscous and flows through the fluid passages in the brake circuit at a significantly slower rate, making it difficult to get the controlled braking pump primed quickly enough to provide adequate braking pressure during a controlled braking event, such as VDC during a quick swerving maneuver, for example.  
       [0011] Prior controlled braking systems ‘A’, such as the one shown in FIG. 1, have incorporated a pre-charge circuit connected in parallel with the master cylinder, and in series with the hydraulic brake circuit to provide a pressurized flow of brake fluid through the hydraulic brake circuit lines to the inlet of the controlled braking pump under cold ambient operating conditions.  
       [0012] The prior controlled braking system A shown in FIG. 1 includes a master cylinder  12 , as shown in FIG. 2, having a cylinder bore  14 , a fluid reservoir  16  for brake fluid, a bleed port  18  providing fluid communication between the cylinder bore  14  and the fluid reservoir  16 , and a primary piston  20  movable in the bore  14  for closing off the bleed port  18  and generating hydraulic braking pressure in the bore  14 .  
       [0013] A primary hydraulic brake circuit  22 , indicated by dashed lines in FIG. 1, is connected in a series fluid circuit relationship to the bore  14  of the master cylinder  12  for delivering pressurized brake fluid at the braking pressure to an inlet/outlet  24  of a hydraulically actuated braking device  26  and receiving a return flow of brake fluid from an inlet/outlet  24  of the hydraulically actuated braking device  26 . It will be noted that the brake system A shown in FIG. 1 includes a primary hydraulic braking circuit  22  including a number of components for controlling the flow and pressure of brake fluid to both a left and right front braking device LF, RF, and a secondary braking circuit, generally indicated by arrow  28 , for controlling both a left and right rear braking device LR, RR.  
       [0014] Because the components in the primary and secondary braking circuits  22 ,  28  are generally identical, only the components in the primary circuit  22  will be described in detail below. It will also be noted that in the system A shown in FIG. 1 the inlet and outlet to the braking device  26  is indicated by a single line  24 . It should be noted that, although FIG. 1 shows a front/ rear split hydraulic system, the same basic circuit arrangement may also be utilized for a diagonal split hydraulic system where, for example, the RF and LR braking devices are connected to the master cylinder primary hydraulic outlet circuit  66  and the LF and RR braking devices are connected to the secondary master cylinder hydraulic outlet circuit  28 .  
       [0015] The primary hydraulic brake circuit  22  includes a normally open inlet control valve  30  and a normally closed outlet control valve  32  for controlling flow in and out of each of the LF and RF braking devices  26 . Each of the control valves  30 ,  32  also has associated therewith a check valve  34  allowing reverse flow, in a direction toward master cylinder  12 , through the check valve  34  when its associated control valve  30 ,  32  is in the closed position. The primary brake circuit also includes an isolation valve  36  and an associated check valve  38 , allowing flow through the check valve  38  in a forward direction, toward the braking device  26 , when the isolation valve  36  is in the closed position. The isolation valve  36  is utilized for regulating or closing off the flow of brake fluid through a portion of the hydraulic brake circuit  22  from the master cylinder  12  during certain controlled braking operations.  
       [0016] A controlled braking pump  40  has an inlet  42  operatively connected through a check valve  46  for receiving brake fluid from the primary hydraulic brake circuit  22 . An accumulator  48  is also connected to both the primary brake circuit and the check valve  46  for storing brake fluid received from the primary brake circuit  22 , and delivering the stored brake fluid through the check valve  46  to the inlet  42  of the controlled braking pump  40 . The controlled braking pump  40  also includes an outlet  44  operatively connected through a damper  50  and an orifice  52  for providing pressurized brake fluid to the hydraulic brake circuit  22  at the braking pressure. The controlled braking pump  40  is driven by a motor  54 .  
       [0017] From the description above, it will be evident that controlled braking systems include a substantial number of components, connected by a number of complex shaped hydraulic conduits or internal passages within the components. At cold operating temperatures, the brake fluid becomes thick and viscous, and it can be difficult for the controlled braking pump  40  to generate a high enough suction at its inlet to quickly generate an adequate flow of fluid for controlled braking operation. To address this problem, some prior controlled braking systems have included a pre-charge circuit, including a pre-charge pump, to help supply an adequate flow of fluid to the inlet of the controlled braking pump  40  at all operating temperatures, for priming the controlled braking pump  40 .  
       [0018] The prior controlled braking system A, shown in FIG. 1, incorporates a pre-charge circuit, generally indicated by arrow  56 , including a pre-charge pump  58  having an inlet  60  operatively connected to the fluid reservoir  16 , and an outlet  62  operatively connected via an outlet check valve  64  to an inlet portion  66  of the primary hydraulic circuit  22 , with the inlet portion  66  being further connected to bore  14  of the master cylinder  12 . The outlet check valve  64  is required, between pre-charge pump  58  and primary hydraulic brake circuit  22 , to prevent reverse flow of fluid from the primary circuit  22  through the pre-charge pump  58  and pressure relief valve  76 .  
       [0019] As will be seen from FIG. 2, the master cylinder  12  also includes a secondary piston  68  and a secondary bleed port  70  in the bore  14 . The secondary piston  68  is separated axially from the primary piston  20  by a space, indicated by arrow  72 , between the primary and secondary pistons  20 ,  68 . The connections between the outlet  62  of the pre-charge pump  58  and the inlet portion  66  of the primary hydraulic brake circuit  22  are made to the bore  14  in the space  72  between the primary and secondary pistons  20 ,  68 , such that pre-charge pressure generated by the pre-charge pump  58  will be communicated directly to the primary hydraulic brake circuit  22  via the inlet  66 , and indirectly to the secondary hydraulic brake circuit  28  by movement of the secondary piston  68  in the bore caused by the existence of the pre-charge pressure in the space  72  between the primary and secondary pistons  20 ,  68  generating an axially acting force on the secondary piston  68 . Some of the flow from the pre-charge pump  58  is lost though primary bleed orifice  18 , but the pre-charge pump  58  is adequately sized to allow for this loss.  
       [0020] The pre-charge circuit  56  also includes a second motor  74  driving the pre-charge pump  58 , and a pressure relief valve  76  connected between the inlet to the outlet  60 ,  62  of the pre-charge pump  58  for regulating the pre-charge pressure generated by the pre-charge pump  58 .  
       [0021] The pre-charge circuit  56  also includes a prime valve  78 , having an inlet  80  connected to the input  66  of the primary hydraulic brake circuit  22 , and an outlet  82  connected to the inlet  42  of the controlled braking pump  40 , for selectively blocking and allowing a flow of brake fluid through the prime valve  78  between the inlet  66  of the primary hydraulic circuit  22  and the inlet  42  of the controlled braking pump  40 . A check valve  84  allows reverse flow of brake fluid through the check valve  84  when the prime valve  78  is blocking flow.  
       [0022] The brake apparatus A also includes a control circuit  86  including a number of sensors, and a control unit (ECU) for sensing when the brake apparatus A should be operated in a controlled braking mode, and connections to the control elements of the apparatus for controlling these elements during the controlled braking operation.  
       [0023] Prior brake systems of the type described above have several drawbacks. The pre-charge pump  58  and its associated drive motor  74 , pressure relief valve  76 , and check valve  64  are typically mounted in a separate, remote, underhood location, which adds a large undesirable volume, on the order of about 500- 1000 cubic centimeters to the brake system, as well as undesirable additional weight, complexity and cost. Also, because the prime valve  78  and its associated check valve  84  are connected to the bore  14  and the inlet  66  to the primary hydraulic circuit  22 , they must be designed to withstand and operate at typical braking pressures of about 2000 pounds per square inch. The need to withstand and operate at maximum braking pressures significantly increases the size, weight and cost of the prime valve  78 .  
       [0024] Commonly assigned U.S. Utility patent application Ser. No. 10/174,601, titled BRAKE SYSTEM FOR A VEHICLE HAVING AN INTEGRAL PRECHARGE PUMP, by Reuter, et al, filed on Jun. 19, 2002 provides a controlled braking apparatus that resolves a number of the problems with prior controlled braking systems having pre-charge circuits, as described above.  
       [0025] Although controlled braking systems practicing the teachings of Reuter, et al, Ser. No. 10/174,601, provide considerable improvement over prior controlled braking systems further improvement is desirable.  
       [0026] In particular, it is desired that back-flow protection be provided integrally within the pre-charge circuits of brake systems according to Reuter, et al, Ser. No. 10/174,601, so that an open return path back to the fluid reservoir 16 is not provided by the pre-charge circuit 56, if one of the outlet control valves 32 fails in an open condition. It is also desirable in hybrid controlled brake systems ‘C’ having a pedal feel emulator 100 according to Reuter, et al, Ser. No. 10/174,601, as shown in FIG. 1C, that the pedal feel emulator  100  not become active until there is a failure of the primary hydraulic brake circuit.  
       [0027] What is needed, therefore, is an improved controlled brake apparatus resolving one or more of the problems identified above. It is also desired that the improved brake apparatus be applicable to hybrid as well as conventional totally hydraulic controlled brake systems.  
       SUMMARY OF THE INVENTION  
       [0028] Our invention provides an improved controlled brake apparatus, meeting the requirements discussed above, through use of a pre-charge circuit connected directly between the fluid reservoir of the master cylinder and the inlet of the controlled braking pump, to thereby direct pre-charge flow and pressure to the controlled braking pump inlet in a parallel circuit relationship to the primary hydraulic circuit, rather than in a series flow arrangement through the primary hydraulic circuit as was the case in prior brake systems. The pre-charge circuit includes an integral back-flow check valve, and may also include an accumulator. In some embodiments of our invention, incorporation of the back-flow check valve and accumulator into the pre-charge circuit allows the elimination of an inlet check valve and accumulator traditionally provided in prior controlled braking systems.  
       [0029] By feeding the pre-charge pressure and flow to the inlet of the controlled braking pump in this parallel circuit manner, components of the pre-charge circuit, such as the prime valve and its associated check valve are not exposed to braking pressure and can be made significantly smaller, lighter and at lower cost. Re-locating the accumulator from the base controlled brake system to the. pre-charge circuit allows the accumulator to be used for fluid storage by both the primary hydraulic circuit and by the pre-charge circuit, to thereby aid in priming the controlled braking pump.  
       [0030] In one form of our invention, a brake apparatus includes a master cylinder, a hydraulic brake circuit, a controlled braking pump and a pre-charge circuit. The master cylinder has a cylinder bore, a fluid reservoir for brake fluid, a bleed port providing fluid communication between the cylinder bore and the fluid reservoir, and a primary piston movable in the bore for closing off the bleed port. and generating hydraulic braking pressure in the bore. The hydraulic brake circuit is connected in a series fluid circuit relationship to the bore of the master cylinder for delivering pressurized brake fluid at the braking pressure to an inlet of a hydraulically actuated braking device and receiving a return flow of brake fluid from an outlet of the hydraulically actuated braking device. The controlled braking pump has an inlet for receiving brake fluid from the hydraulic brake circuit and an outlet for providing pressurized brake fluid to the hydraulic brake circuit at the braking pressure required by the vehicle.  
       [0031] The pre-charge circuit has an inlet operatively connected to the fluid reservoir and an outlet operatively connected to the inlet of the controlled braking pump to thereby form a parallel circuit to the series circuit formed by the cylinder bore and the hydraulic brake circuit for providing a pre-charge flow of brake fluid to the inlet of the controlled braking pump at a pre-charge pressure. The pre-charge circuit includes a back-flow check valve for allowing the pre-charge flow of brake fluid to pass through the back-flow check valve from the fluid reservoir to the inlet of the controlled braking pump and blocking a flow of brake fluid from the inlet of the controlled braking pump to the fluid reservoir. The pre-charge circuit may also include a pressure relief valve or equivalent flow restriction device for regulation of the pre-charge pressure.  
       [0032] The pre-charge circuit may include a pre-charge pump having an inlet operatively connected to the fluid reservoir and an outlet operatively connected to the inlet of the controlled braking pump. The pre-charge circuit may also include a prime valve operatively connected within the pre-charge circuit in a series circuit relationship between the outlet of the pre-charge pump and the inlet of the controlled braking pump for selectively blocking or passing a flow of brake fluid through the prime valve.  
       [0033] In another form of our invention, a brake apparatus includes a hydraulic brake circuit, a controlled braking pump having an inlet for receiving brake fluid from the hydraulic brake circuit and an outlet for providing pressurized brake fluid to the hydraulic brake circuit at a braking pressure, a motor-driving the controlled braking pump, and a pre-charge pump driven by the motor for providing a priming flow of brake fluid to the inlet of the controlled braking pump at a charge pressure lower than the braking pressure. A back-flow check valve is operatively connected between the inlet of the controlled braking pump and the outlet of the pre-charge pump for allowing the priming flow of brake fluid to pass through the back-flow check valve from the pre-charge pump to the inlet of the controlled braking pump and blocking a flow of brake fluid from the inlet of the controlled braking pump to the pre-charge pump.  
       [0034] In some forms of our invention the master cylinder of a brake apparatus includes a second piston in the bore separated from the first piston to form a primary fluid cavity in the bore between the first and second pistons connected in fluid communication to the hydraulic brake circuit, and a secondary fluid cavity between the second piston and the bore. The brake system also includes a pedal feel emulator, that provides a near-normal pedal movement, as perceived by the driver, if the primary hydraulic system experiences a problem that would otherwise alter the feel of the pedal. The brake pedal feel emulator has a first and a second fluid cavity on opposite sides of a movable wall. The first fluid cavity of the pedal feel emulator is connected in fluid communication to the secondary fluid cavity of the master cylinder, and the second fluid cavity of the pedal feel emulator connected in fluid communication to the primary fluid cavity of the master cylinder, to thereby provide a hydraulic lock against movement of the movable wall of the pedal feel emulator until hydraulic pressure is lost in the primary fluid cavity of the master cylinder.  
       [0035] Our invention may also take the form of a method for operating a brake using the apparatus described herein, and is applicable to a variety of controlled braking systems including hydraulic, hybrid and electrically actuated braking devices including electromechanical and electro-hydraulic brake devices.  
       [0036] The foregoing and other features and advantages of our invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of our invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0037]FIG. 1 is a schematic representation of a prior controlled brake system;  
     [0038]FIG. 2 is a cross section of a typical master cylinder of the type used in the prior brake system of FIG. 1, and in exemplary embodiments of brake systems, according to our invention, as shown in FIGS.  3 - 10 ;  
     [0039]FIG. 3 is a schematic of a first exemplary embodiment of a brake apparatus according to our invention; and  
     [0040] FIGS.  4 - 10  are schematic representations of alternate embodiments of our invention. 
    
    
     DETAILED DESCRIPTION  
     [0041]FIG. 3 depicts a first exemplary form of a brake apparatus  10 , according to our invention. The embodiment depicted in FIG. 3 includes many of the same components as the prior apparatus A discussed above with regard to FIG. 1 and FIG. 2. To facilitate understanding of our invention and the distinctions between our invention and prior brake systems, the components in FIG. 3 and subsequent FIGS.  4 - 10  that are substantially equivalent or similar to the components described above, in the Background of the Invention, will be given the same reference numbers used in the Background of the Invention. For the sake of brevity, where components bearing similar reference numbers are substantially equivalent or similar in function to the components described in detail above, the description of those components and their functions will not be repeated below. The exemplary embodiment of the brake apparatus  10  depicted in FIG. 3, includes a master cylinder  12  similar to the one depicted in FIG. 2, having a cylinder bore  14 , a fluid reservoir  16  for brake fluid, a bleed port  18  providing fluid communication between the cylinder bore  14  and the fluid reservoir  16 , and a primary piston  20  movable in the bore  14  for closing off the bleed port  18  and generating hydraulic braking pressure in the bore  14 .  
     [0042] A hydraulic brake circuit  22 , indicated by dashed lines, is connected in a series fluid circuit relationship to the bore  14  of the master cylinder  12  for delivering pressurized brake fluid at the braking pressure to an inlet/outlet  24 . of a hydraulically actuated braking device, and for receiving a return flow of brake fluid from the inlet/outlet  24  of the hydraulically actuated braking device  22 .  
     [0043] A controlled braking pump  40  has an inlet  42  for receiving brake fluid from the hydraulic brake circuit  22  and an outlet  44  for providing pressurized brake fluid to the hydraulic brake circuit  22  at the braking pressure.  
     [0044] In contrast to the prior brake apparatus A depicted in FIG. 1, however, a pre-charge circuit  88  has an inlet  90  operatively connected to the fluid reservoir  16 , and an outlet  92  operatively connected to the inlet  42  of the controlled braking pump  40 , to thereby form a parallel circuit to the series circuit formed by the cylinder bore  14  and the hydraulic brake circuit  22  for providing a pre-charge flow of brake fluid to the inlet  42  of the controlled braking pump  40  at a pre-charge pressure.  
     [0045] The pre-charge circuit  88  includes a back-flow check valve  46  for allowing the pre-charge flow of brake fluid to pass through the back-flow check valve  46  from the fluid reservoir  16  to the inlet  42  of the controlled braking pump  40  and blocking a flow of brake fluid from the inlet  42  of the controlled braking pump  42  to the fluid reservoir  16 .  
     [0046] The brake apparatus  1   0  further includes an accumulator  48  commonly connected in fluid communication with the inlet  42  of the controlled braking pump  40 , the inlet/outlet  24  of the hydraulically actuated braking devices  26 , and the outlet  92  of the pre-charge circuit  88 . The connection to the inlet/outlet  24  of the hydraulically actuated braking devices  26  is provided via an outlet  27  of the primary braking circuit  22  flowing through the series-connected control valve  32 .  
     [0047] The pre-charge circuit  88  further includes a pre-charge pump  58  having an inlet  60  operatively connected to the fluid reservoir  16 , and an outlet  62  operatively connected to the inlet  42  of the controlled braking pump  40 . Specifically, the outlet  62  of the pre-charge pump  58  is connected in a series circuit relationship to the inlet  80  of a prime valve  78 , and the outlet  82  of the prime valve  78  is connected in a series circuit relationship through the back-flow check valve  46  to the inlet  42  of the controlled braking pump  40 , for selectively blocking or passing a flow of brake fluid through the prime valve  78 .  
     [0048] Also in contrast to the prior brake apparatus A shown in FIG. 1, the brake apparatus  10  shown in FIG. 3 includes a motor  54  operatively connected for driving both the controlled braking pump  40  and the pre-charge pump  58 .  
     [0049] The pre-charge circuit  88  shown in FIG. 3 further includes a pressure operated relief valve  76  operatively connected from the outlet to the inlet of the pre-charge pump  58 . A bleed orifice  98 , of small diameter, on the order of 0.010 inch, is connected in parallel to the pressure operated relief valve  76  to facilitate bleeding air from the controlled braking system  10  to facilitate filling the braking system  10  with hydraulic brake fluid, utilizing the commonly practiced evacuation and filling techniques employed in many vehicle assembly plants.  
     [0050] The pre-charge circuit  88  further includes a check valve  84  operatively connected from the outlet  82  of the prime valve  78  to the outlet  44  of the pre-charge pump  40  for blocking flow through the check valve  84  from the inlet to the outlet  80 ,  82  of the prime valve  78 , and allowing flow through the check valve  84  from the outlet  82  to the inlet  80  of the prime valve  78 .  
     [0051]FIG. 4 shows an alternate exemplary embodiment of a brake apparatus  10  according to our invention, having a second pre-charge circuit  94  connected between the fluid reservoir  16  and the inlet  96  of the controlled braking pump  40  of the secondary hydraulic braking circuit  28 . The second pre-charge circuit  94  is identical in all other respects to the pre-charge circuit  88  described above. The second pre-charge circuit also includes a second pre-charge pump  58  driven by the same motor  54  used for driving the controlled braking pumps  40  in the primary and secondary braking circuits  22 ,  28 .  
     [0052]FIG. 5 shows a hybrid. brake apparatus  10  according to our invention, having front hydraulic brakes LF, RF, and electrically actuated rear brakes LR, RR. The hybrid brake apparatus  10  includes a pre-charge circuit  88  as described above, with the motor  54  driving both the controlled braking pump  40  and the pre-charge pump  58 . The secondary hydraulic circuit  28  includes a pedal-feel emulator  100  and a pressure sensor  102  operatively connected to receive pressurized brake fluid from the secondary piston  68  of the master cylinder  12 .  
     [0053] As shown in FIG. 2, the master cylinder  12  in the exemplary embodiment of the brake system  10  shown in FIG. 5, includes a secondary piston  68  in the cylinder bore  14 , separated from the first piston  20  to form a primary fluid cavity  72  in the bore  14  between the primary and secondary pistons  20 ,  68 . The primary fluid cavity is connected in fluid communication through a primary port  13  of the master cylinder  12  to the hydraulic brake circuit  22 . The master cylinder  12  also includes a secondary fluid cavity  73  between the second piston  68  and the bore  14 , and connected to a secondary fluid port  15  of the master cylinder  12 .  
     [0054] A brake pedal feel emulator  100 , according to the present invention, has a first and a second fluid cavity  104 ,  106  on opposite sides of a movable wall  108 . The first fluid cavity  104  of the pedal feel emulator  100  is connected in fluid communication to the secondary fluid cavity  73  of the master cylinder  12 , through the secondary fluid port  15  of the master cylinder 12 . The second fluid cavity  106  of the pedal feel emulator  100  is connected in fluid communication to the primary fluid cavity  72  of the master cylinder  12 , through the primary fluid port  13  of the master cylinder  12 . By virtue of adding the second fluid cavity  106  in the pedal feel emulator  100  of the present invention, connected in fluid communication with the primary fluid cavity  72  of the master cylinder  12 , as shown in FIG. 5, the pressure existing in the primary cavity  72  of the master cylinder is applied directly to the back side of the movable wall  108 , and indirectly to the front side (first fluid cavity  104 ) of the pedal feel emulator  100 , to thereby provide a hydraulic lock against movement of the movable wall  108  of the pedal feel emulator  100  until hydraulic pressure is lost in the primary fluid cavity  72  of the master cylinder  12 . Thus, when the primary circuit is functional, there are no additional travel losses occurring from the pedal feel emulator  108 .  
     [0055] Once pressure in the primary cavity  72  is lost, the pressure on the back side of the movable wall  108  of the pedal feel emulator  100  will drop to atmospheric pressure, and the primary piston  20  will move forward under pedal pressure to a lower pedal position, whereas the primary piston  20  will bear against the secondary piston  68 . Further pedal movement will then move the secondary piston  68  in the bore  14  to expel fluid from the secondary fluid cavity  73  of the master cylinder  12  into the pedal feel emulator  100 . A spring mechanism  110  in the pedal feel emulator  100  resists movement of the movable wall  108  of the pedal feel emulator  100 , to simulate normal pedal feel to the operator, albeit at a lower pedal height than normal.  
     [0056]FIG. 6 shows an embodiment of a brake apparatus  10 , according to our invention, having a pre-charge pump  58  driven by a common motor  54  with a pair of controlled braking pumps  40 , in the same manner as described above with regard to the embodiment of FIG. 3, but having the output  62  of the pre-charge pump and the inlet  80  of a prime valve  78  connected to the inlet  66  of the hydraulic brake circuit  22  and bore  14 , as described above in relation to FIGS. 1 and 2. An outlet check valve  64  is located between pre-charge pump  58  and primary hydraulic brake circuit  22  to prevent reverse flow of fluid from the primary circuit  22  through the pre-charge pump  58  and pressure relief valve  76 .  
     [0057]FIG. 7 shows an embodiment of our brake apparatus  10  that is similar to the embodiment in FIG. 6, but having a second pre-charge pump  58  and a second prime valve  78  connected in the same manner as the first pre-charge pump and prime valve  58 ,  78  described above with respect to FIG. 6, and both pre-charge pumps  58  being driven in common by motor  54  with two controlled braking pumps  40 .  
     [0058]FIG. 8 shows an embodiment of our brake apparatus  10  that includes a pair of prime valves  78  having their inputs  80  connected to the output  62  of the pre-charge pump  58 , as described above in relation to FIG. 3 rather than to. the inlet  66  of the hydraulic brake circuit  22  and bore  14  as described in regard to the embodiment depicted in FIGS. 1 and 2, but having the pre-charge pump  58  driven by a second motor  74 , rather than the motor  54  driving a pair of controlled braking pumps  40 .  
     [0059] Those having skill in the art will recognize that, while we presently consider it preferable to have the. components according to our invention arranged as described above, we contemplate many other arrangements within the scope of our invention.  
     [0060] For example, FIGS. 9 and 10 show embodiments of our invention in hybrid brake systems having the pre-charge circuit in series with the primary brake circuit, in a manner similar to the embodiment shown in FIG. 1, but having a pedal feel emulator  100  according to the present invention, as described above with regard to FIG. 5.  
     [0061]FIG. 9 shows a hybrid brake apparatus  10 , according to our invention having a pre-charge pump  58  and a controlled braking pump  40  both driven by a common motor  54 , as described above in relation to FIG. 5, but having the output  62  of the pre-charge pump  58  and the inlet  80  of a prime valve  78  connected to the inlet  66  of the hydraulic brake circuit  22  and bore  14 , as described above in relation to FIGS. 1 and 2.  
     [0062]FIG. 10 shows a hybrid brake apparatus  10 , according. to our invention, having the input  80  of a prime valve  78  connected to the output  62  of the pre-charge pump  58 , as described above in relation to FIG. 3 rather than to the inlet  66  of the hydraulic brake circuit  22  and bore  14  as described in regard to the embodiment depicted in FIGS. 1 and 2, but having the pre-charge pump  58  driven by a second motor  74 , rather than the motor  54  driving the controlled braking pump  40 .  
     [0063] It is not necessary in the embodiments of FIGS. 6, 7, or  9  to provide a back-flow check valve  46  in the pre-charge circuit, as was the case in the exemplary embodiments shown in FIGS.  3 - 5 ,  8 , and  10  to avoid having an open return path to master cylinder reservoir  16 , since the output of the pre-charge pump  58  in those cases empties directly into primary master cylinder circuit  66 .  
     [0064] We also contemplate that the hydraulic locking method and apparatus of our invention, as disclosed above with regard to the embodiments shown in FIGS. 5 and 9, may be utilized for locking a pedal feel emulator against movement until hydraulic pressure is lost in the primary brake circuit in all hydraulic or other types of brake systems that may or may not include a pre-charge circuit.  
     [0065] In summary therefore, while the embodiments of our invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. The scope of the invention is indicated in the appended claims, and all changes or modifications within the meaning and range of equivalents are intended to be embraced therein.