Vehicle braking system master cylinder

A master cylinder suitable for a vehicle braking system has an unusual capability to supply quantities of fluid and fluid pressures at its output in differentiated manner in dependence on the braking stage, preventing long transitory stages between an approach stage and an actual braking stage. The cylinder comprises a cylinder body having a first chamber and a second chamber provided inside. The first chamber is in operative communication with at least one braking member and houses a first piston. The second chamber, which has larger transverse dimensions than the first chamber, is in operative communication with the first chamber and houses a second piston. Ducts are provided and put the first chamber and the second chamber into communication during the approach stage. The ducts house a low-pressure valve which is open during the approach stage and closed during the actual braking stage.

FIELD OF THE INVENTION

The subject of the present invention is a master cylinder for a vehicle braking system.

BACKGROUND OF THE INVENTION

The present invention relates particularly to a so-called differential cylinder, that is, a cylinder which enables the amount of fluid at the output and its pressure to be varied in dependence on the braking stage, as will be described below and as described, for example in U.S. Pat. No. 4,455,831, GB 359142, DE 2436808, U.S. Pat. No. 2,093,543, DE 3631683 and U.S. Pat. No. 1,958,722.

As is self-evident, braking systems operate at least between a rest condition and a working condition which are defined, respectively, by the absence or by the presence of a predetermined braking force applied to the members of the system by actuating means.

Irrespective of the type of braking system, the transition from the rest condition to the working condition comprises a first stage for taking up clearance and a second actual braking stage.

With reference, for example, to hydraulically-operated braking systems, during the stage for taking up clearance, the travel performed by the actuating means is quite large and the pressure is low. In contrast, during the actual braking stage, the travel performed by the actuating means is quite small and the pressure is high.

With reference to braking systems of the type with disc brakes, a disc is mounted on the hub of the vehicle wheel and a caliper, provided with pads, is mounted on a fixed portion of the vehicle, for example, on the suspension, and is arranged astride the disc. By way of example, for a so-called fixed caliper, the caliper has cylinder-piston units disposed on both sides of the disc so that the respective pistons act on the pads, pressing them against the disc to achieve the desired braking action.

When the braking system is in the rest condition, the pads are disposed at a distance from the disc such as to allow free rotation of the disc and not to give rise to so-called residual-torque phenomena. Upon completion of the braking operation, the pistons are therefore retracted inside the cylinders by a distance, generally known by the term “rollback”, such as to permit the desired detachment of the pads from the disc.

It is therefore clear that, during the transition from the rest condition to the working condition of the braking system, the pistons which act on the pads perform, in the first place, a travel which causes the pads to approach the surface of the disc, that is, a travel during which the distance (the rollback) between the pads and the disc is taken up.

In these braking systems, the cylinder-piston units of the caliper are arranged to be connected to a master cylinder which in turn is connected to a control device, for example, constituted by the brake pedal provided inside the passenger compartment.

The approach travel of the pads in order to exert the braking force on the vehicle wheels translates into an approach travel of the master cylinder, that is, into a travel which is lost for the purposes of the braking action. Similarly, the approach travel of the master cylinder translates into a corresponding travel of the brake pedal, during which the pedal does not generate the desired braking force.

Naturally, the approach travel of the pads, together with the consequent loadless travel of the pedal master cylinder, adversely affect the promptness of the response of the brake and this effect is more noticeable the greater the sensitivity of the user and the higher the performance of the vehicle, as in racing cars or, in any case, vehicles for sports uses.

There is therefore a need, on the one hand, to limit the approach travel of the pedal whilst nevertheless supplying a large amount of fluid to the cylinder-piston units housed in the caliper. This need could be solved by providing a master cylinder with a large-diameter piston. On the other hand, however, there is a need to limit the force which must be applied to the brake pedal in order to achieve a predetermined pressure. This need could be solved by providing a master cylinder with a piston of limited diameter.

These conflicting needs are solved simultaneously by providing a master cylinder, known as a differential cylinder, which has a cylindrical cavity having two portions with different diameters, in each of which a corresponding piston slides.

This cylinder therefore has two chambers with different diameters for performing two distinct functions. During the stage of the approach travel of the pads, the larger-diameter chamber is operative and can supply a large quantity of fluid to the brakes whilst keeping the travel of the brake pedal short. During the actual braking stage, the smaller-diameter chamber is operative and enables high pressures of the fluid to be achieved and large braking forces therefore to be exerted whilst the control force on the brake pedal is kept low. The transition from one function to the other takes place when a predetermined pressure value (for example 5 bar) is reached, and is brought about by the opening of a discharge valve by means of which the larger-diameter chamber is put into communication with ambient pressure, for example, from the brake fluid reservoir. An example of a differential cylinder as described briefly above is given, for example, in U.S. Pat. No. 1,958,722. The larger-diameter chamber is put into communication with the reservoir by means of ducts formed in the structure of the cylinder body. The discharge valve is housed inside the ducts, and hence inside the body of the master cylinder.

Further constructions, in which the discharge valve which renders the larger-diameter chamber inactive is housed inside the pistons, for example, inside the rod of the larger-diameter piston, are described and illustrated in GB 359142, DE 2436808, U.S. Pat. No. 2,093,543 and DE 3631683.

The above-mentioned patents describe differential cylinders in which the smaller-diameter piston has some ducts parallel to the axis of sliding of the pistons. Moreover, the seal between the smaller-diameter piston and the respective chamber is ensured by a lipped seal which adheres to the walls of the chamber as a result of the pressure established therein.

During the stage in which the larger-diameter chamber is active, it is known, for example, as described in the patents cited above, for the brake fluid to pass from the larger-diameter chamber to the smaller-diameter chamber through the ducts formed in the smaller-diameter piston and between the walls and the lipped seal, which is not yet adhering to the walls.

When the “rollback” and the clearance have been taken up, the larger-diameter piston is deactivated by the opening of the discharge valve and the lipped seal of the smaller-diameter piston adheres to the walls of its chamber which is isolated from the larger-diameter chamber, permitting an increase in the pressure which causes the pads to be thrust against the disc in order to exert the braking force on the vehicle.

The transitory stage between the deactivation of the larger-diameter chamber and the activation of the smaller-diameter chamber has to be kept to the minimum since it corresponds to an “idle” period of time which could instead be used for the delivery of brake fluid under pressure.

Moreover, the duration of the transitory stage depends on the promptness of the response of the lipped seal which, unfortunately, is not optimal, because it is also designed so as to be suitable for forming the seal against the walls of the respective chamber. This translates into a sensation in the brake pedal of a “step”, which is particularly disagreeable for sports cars or even racing cars.

A further disadvantage of known cylinders arises when, owing to the pressure established in the smaller-diameter chamber, the lipped seal is extruded through the axial holes in the smaller-diameter piston. Attempts have been made to prevent this problem by inserting a ring between the lipped seal and the respective piston, but the solution is not optimal since it is a further factor which contributes to increase “idle” time.

It is known from U.S. Pat. No. 4,455,831 to produce a differential cylinder provided with valve means which are separate from the lipped seal of the smaller-diameter piston and which put the two chambers into communication.

Finally, a two-part piston inside which two separate valves are housed is known from U.S. Pat. No. 3,667,229. A first valve is disposed in a duct inside the piston, which connects the smaller-diameter chamber with the larger-diameter chamber. A second valve is disposed in a duct which connects the larger-diameter chamber with an environment that is in communication with the reservoir. The piston is provided in two portions and each houses a respective valve.

The two valves are separate from one another and each has to be mounted, separately from the other, inside the respective portion of the piston. This leads to problems in the production of the two portions, both of which have to be precision machined in order to house the respective valve correctly, and to problems during assembly, in particular rendering assembly quite complex.

The object of the present invention is to devise and to provide a master cylinder for a vehicle braking system which permits differential operation and, at the same time, overcomes the disadvantages mentioned with reference to the prior art.

SUMMARY OF THE INVENTION

Objects according to various embodiments of the present invention are achieved by means of a master cylinder for a vehicle braking system. The master cylinder comprises a cylinder body having a first chamber and a second chamber provided inside. The first chamber is in operative communication with at least one braking member and houses a first piston. The second chamber, which has larger transverse dimensions than the first chamber, is in operative communication with the first chamber and houses a second piston. Ducts are provided and put the first chamber and the second chamber into communication during the approach stage. The ducts house a low-pressure valve which is open during the approach stage and closed during the actual braking stage.

DETAILED DESCRIPTION OF THE INVENTION

With reference to the above-mentioned drawings and in particular toFIG. 1, a master cylinder of a vehicle braking system is generally indicated10.

The cylinder10comprises a body12inside which there is a cavity14having one open end and the other end closed by a base16.

The cavity14extends along a longitudinal axis18and has cylindrical portions with different diameters. In the embodiment ofFIG. 1, the cavity14has substantially a first cylindrical portion which extends from the base16and a second cylindrical portion which extends from the first portion to the free end of the cavity. The first portion has a slightly smaller diameter than the second portion.

The body12has an inlet duct20and an output duct22which are put into communication with the interior of the cavity14and, in particular, with the second, larger-diameter portion and with the first, smaller diameter portion, respectively. According to a possible embodiment, both of the ducts preferably extend substantially radially relative to the body12. Moreover, the output duct22is formed in the vicinity of the base16.

According to a possible embodiment, in the region of the inlet duct20, the body12has a seat24suitable for housing appropriate elements for connection to a brake-fluid reservoir, not shown. Similarly, according to one embodiment, in the region of the output duct22, the body12has a further seat26which may be suitably threaded and which can be put into communication with actuating means, not shown, for example, constituted by cylinder-piston units of disc-brake calipers.

According to a possible embodiment, the body12comprises at least one further duct28suitable for putting the seat24and the second portion of the cavity14into communication.

According to a possible embodiment shown inFIG. 1, a cylindrical bush30, preferably suitable for abutting the surface of the base16inside the body12, is inserted in the cavity14, coaxially therewith. The bush30is perforated axially and extends along the first portion of the cavity14, further reducing its diameter inside the body12.

According to a possible embodiment, the wall of the bush has radial holes32distributed around a circumferential portion in the vicinity of the end which abuts the surface of the base16.

The outside diameter of the bush30is reduced in the region of the circumferential portion in which the radial holes32are distributed. A small annular chamber33is thus created between the walls of the cavity14and the bush30in the region of the radial holes32, and is put into communication with the output duct22. According to a possible embodiment, starting from the end which is intended to abut the base16, the outer surface of the bush30also has cylindrical sealing portions and threaded portions for cooperating with corresponding threaded portions formed in the surface of the first portion of the cavity14.

In the embodiment ofFIG. 1, a first cylindrical sealing portion of the bush30is intended to cooperate with a sealing ring34associated with an anti-extrusion ring36, both housed in a seat38in the cylinder body. According to a possible embodiment, the bush30comprises a second cylindrical sealing portion provided with a seat40suitable for housing a further sealing ring42.

According to a possible embodiment, the end of the bush30remote from that which is intended to abut the base16has some axial grooves44formed in the outer surface of the bush30, that is, in the surface which directly faces the surface of the cavity14. The grooves44extend longitudinally from the end of the bush30remote from that which is intended to abut the base16and are put into communication with the interior of the bush30by means of radial holes46.

In the embodiment ofFIG. 1, four grooves are provided, and are distributed uniformly circumferentially.

A first piston for sliding inside the bush30is indicated48and a second piston for sliding inside the second, larger-diameter portion of the cavity14, is, indicated50.

According to a possible embodiment, the first piston48is formed, in a single piece, by a thrust portion48aand by a rod48b. The thrust portion48ahas a flange-shaped head having transverse dimensions substantially corresponding to the inside transverse dimensions of the bush30.

A seat is also provided for housing sealing means, for example, formed by a first primary seal52, restrained by a ring or by an undercut54.

According to a possible embodiment, the rod48bhas, at the end remote from the thrust portion48a, a portion suitable for coupling with the second piston50.

Externally and in a direction away from the thrust portion48a, this coupling portion comprises a flanged portion, which acts as a shoulder for a threaded portion, and a cylindrical portion. The flanged portion has a seat56for a sealing ring58.

According to a possible embodiment, the rod48bis hollow axially, that is, it has an axial hole60which extends throughout its length and is open at both ends. According to a possible embodiment, the rod48balso comprises ducts61, preferably radial ducts, for example, disposed between the thrust portion48aand said flanged portion. The radial ducts61are put into communication with the axial hole60.

The thrust portion48aof the first piston48, the inside wall of the bush30, and the base16define a first chamber A or high-pressure chamber which opens into the output duct22.

A return spring, indicated66, is housed in the first chamber A and partially inside the axial hole60, operating between the base16and the first piston48. According to a possible embodiment, the return spring66has a guide element68bearing on the surface of the base16.

According to a possible embodiment, the second piston50is formed, in a single piece, by a thrust portion50aand by a rod50b. The thrust portion50acomprises a flange-shaped head having transverse dimensions substantially corresponding to the transverse dimensions of the second portion of the cavity14. Axial through-holes formed in the thrust portion50aof the second piston50are indicated70.

A second primary seal72is housed in a seat of the thrust portion50aof the second piston50, in contact with one end of each axial hole70and arranged in a manner such as to adhere to the walls of the cavity14. According to a possible embodiment, a ring, not shown, may be provided between the primary seal72and the flange-shaped head of the piston, to prevent the seal being extruded through the axial holes70because of the pressure.

According to a possible embodiment, an axial hole74is formed in the head which constitutes the thrust portion50aand has portions with different diameters for housing the coupling portion of the first piston48. According to the embodiment shown inFIG. 1, there may be a first cylindrical portion for cooperating with the sealing ring58of the first piston48, a threaded portion for cooperating with the corresponding threaded portion of the first piston48, and a second cylindrical portion which houses the final cylindrical portion of the first piston48and terminates in a base surface. Some ducts76extend from the second cylindrical portion and through the whole thickness of the wall of the rod50b, opening outside it.

According to a possible embodiment, a shank78is provided at the end remote from the thrust portion50a, for example, a threaded shank for coupling with the control pedal.

According to a possible embodiment, the first piston48and the second piston50are fixed firmly to one another, as shown, for example, inFIG. 1. When the two pistons are assembled, shims79may be interposed between the flanged portion of the first piston and a corresponding shoulder in the axial hole74of the second piston so as to define precisely the position of the two pistons relative to one another. This relative position depends on the distance between the inlet duct20and the radial ducts46.

As shown inFIG. 1, when the two pistons are assembled, the axial hole60opens into the base portion of the axial hole74and is put into communication with the ducts76.

The thrust portion48aof the first piston48, the thrust portion50aof the second piston50, and the inside wall of the second portion of the cavity14define a second chamber B or low-pressure chamber which, as shown inFIG. 1, is put into communication with the inlet duct20.

According to a possible embodiment, first valve means are housed in the axial hole60between the radial ducts61and the end for coupling with the second piston70. In one possible embodiment shown, for example, inFIG. 1, the first valve means comprise a discharge valve80. According to a possible embodiment, the discharge valve80comprises an independent cylinder81suitable for being fitted in the axial hole60and for expanding against the walls of the axial hole, forming an interference coupling. The cylinder81houses a sphere82which defines the valve-closure element and a spring83which keeps the valve closed until a predetermined threshold pressure p′ is reached inside the low-pressure chamber B, preventing brake fluid flowing from the radial ducts61to the ducts76below this value. According to a possible embodiment, the discharge valve is calibrated to open at a pressure equal to a threshold value p′ of about 5 bar.

According to a possible embodiment, further valve means are provided, for example, fitted in the axial hole60. According to a possible embodiment, the further valve means comprise a low-pressure valve84fitted in the axial hole60between the radial ducts61and the high-pressure chamber A.

In one possible embodiment, for example, shown inFIG. 1, the low-pressure valve84comprises an independent cylinder85suitable for being fitted in the axial hole60and for expanding against the walls of the axial hole, forming an interference coupling. According to a possible embodiment, the cylinder85houses a sphere86which defines the valve-closure element and a spring87which keeps the valve in position until a predetermined threshold pressure p″ is reached inside the low-pressure chamber B.

According to a possible embodiment, the low-pressure valve is open from the start, and hence from a substantially ambient pressure value, until the pressure p inside the second chamber reaches the threshold value p′ (5 bar), that is, until the discharge valve80opens and the pressure P inside the first chamber A exceeds the pressure p inside the second chamber B. According to a further embodiment, the low-pressure valve84is calibrated to open as soon as the pressure rises slightly above ambient pressure, for example, to a threshold value p″ of about 0.1 bar, and remains open until the pressure P inside the first chamber A exceeds the pressure p inside the second chamber B.

The low-pressure valve84defines valve means which are separate from the primary seal52and which can put the two chambers A and B into communication in a first stage and can close communication between the two chambers in a second stage.

According to a possible embodiment, a filter88is interposed between the two valves, at the confluence of the radial ducts61.

To complete the differential cylinder10, a ring nut90is screwed into the open end of the cavity14and retains and guides the two pistons. This ring nut has an outer cylindrical portion in which a seat for a sealing ring92is defined, a threaded portion for securing to the cylinder body12, and a collar which forms an abutment shoulder for the end surface of the cylinder body12. When the ring nut is screwed into the cylinder body, shims94can be inserted between the collar of the ring nut and the end surface of the body12.

The ring nut90also provides a seat96for the engagement of a protective boot98which is closed onto the rod of the second piston, and a further seat100for a sealing ring102.

According to a possible embodiment, an internal cylindrical cavity is also provided and has a diameter slightly greater than the diameter of the rod50bof the second piston50so as to form a chamber104. The chamber104is put into communication with the duct28. The chamber104is also put into communication with the ducts76, and hence with the axial hole60and, when the valve80is in the open position, also with the radial ducts61and the second chamber B or low-pressure chamber.

The end of the ring nut which is inside the body12constitutes the stop abutment for the second piston50and, by its position, defines the correct rest position thereof, with the interposition of the shims94inserted between the collar of the ring nut90, which is inside the body12, and the end of the cylinder body.

A possible alternative embodiment of the cylinder is shown in rest conditions inFIG. 2. Elements which are similar to those of the embodiment ofFIG. 1are indicated by the same reference numerals.

The cylinder ofFIG. 2differs from that ofFIG. 1by the presence of an integrated block106housed inside the axial hole60. The integrated block106comprises both the discharge valve80and the low-pressure valve84, which are housed in a single cylindrical body108that can be fitted in the axial hole60. The integrated block106is preferably suitable for being fitted in the first piston48from the end facing the base16, in abutment with shoulders of the axial hole60.

According to a possible embodiment, the integrated block106also comprises the filter88and radial ducts110which can be arranged in the region of the radial ducts61.

According to a possible embodiment, the outer wall of the integrated block106has two seats112, disposed on opposite sides of the radial ducts110and suitable for housing seals114.

According to a possible embodiment, the integrated block106has means for securing to the interior of the piston in which it is housed. A threaded connection116, for, example, with a nut117, is formed at an end of the integrated block106, projecting from the axial hole60. The first piston48is hollow and houses the integrated block completely, allowing it to enter from one end and to be secured from the other. Preferably, as shown inFIG. 2, the integrated block106is inserted in the first piston48from the end facing the base16and is locked therein from the opposite end, that is from the end which is mounted in the second piston50.

The discharge valve and the low-pressure valve may have characteristics similar to those described for the embodiment ofFIG. 1.

Operation is described below with reference toFIGS. 2–5, that is, to a cylinder comprising an integrated block106. The operation of the cylinder ofFIG. 1or of other equivalent variants is similar.

FIG. 2shows the cylinder in the rest position corresponding to the condition in which no force is applied to the brake pedal. In this position, the return spring66urges the unit constituted by the first and second pistons against the ring nut which, as mentioned above, constitutes the stop abutment and hence defines its rest position.

The second chamber B is in communication with the inlet duct20by virtue of the position of the second piston,50and, in particular, of the second primary seal72.

Moreover, the first chamber A is in communication with the radial holes46and the grooves44, by virtue of the position of the first piston48and, in particular, of the first primary seal52.

The low-pressure chamber B and the high-pressure chamber A are therefore in communication with one another and with the reservoir, and are consequently at ambient pressure.

If the brake is operated, that is, if a force is applied to the pedal operatively connected to the shank78, the unit comprising the two pistons is caused to slide towards the base16, along the longitudinal axis18.

As soon as the second primary seal72passes beyond the inlet duct20(FIG. 3), the fluid starts to move towards the output duct22and towards the brake-actuating members, that is, the stage starts in which the pads approach the disc and the so-called “rollback” is taken up.

During this stage, the amount of fluid moved is equal to the cross-section of the second chamber B multiplied by the travel performed by the piston.

According to a possible embodiment, the slight over-pressure p″ which is established in the second chamber B is such as to open the low-pressure valve84so that the fluid passes from the second chamber B to the first chamber A through the radial holes61, the low-pressure valve84, and the axial hole60. Naturally, the fluid also flows from the first chamber A to the brake through the output duct22. According to a possible embodiment, the low-pressure valve is opened immediately, even at a pressure corresponding to ambient pressure.

It is therefore clear that, in this stage, the second piston and the respective second chamber, or low-pressure chamber B are active and that, by virtue of the larger diameter of the piston, it is possible to move larger quantities of fluid, whilst the travel both of the piston and of the brake pedal are kept short.

The chamber104is enlarged since it is now defined by the ring nut90, by the thrust portion50aof the second piston50, and by the walls of the cavity14. The chamber104is in communication with the ducts28and with the inlet duct20, that is, with the brake-fluid reservoir. The pressure inside the chamber104is therefore equal to ambient pressure.

During the approach stage, the pressure in the first and second chambers is equal to a value which is less than the calibrated threshold pressure p′ of the discharge valve80(for example, 5 bar) and which increases as the two pistons translate towards the base16.

The approach stage terminates after a travel of the pistons of about 0.2 mm and corresponds substantially to the moment at which the pads come into contact with the disc and the actual braking stage starts. The start of this stage is characterized by the reaching and the exceeding of the calibration pressure p′ of the discharge valve80(for example, 5 bar).

The opening of the discharge valve80(FIG. 4) puts the second chamber B into communication with the chamber104which is at ambient pressure, thus preventing the pressure in the second chamber B from increasing above the predetermined value. The opening of the discharge valve80thus creates a pressure difference between the second chamber B and the first chamber A, which tends to close the low-pressure valve84promptly.

The opening of the discharge valve80and the closure of the low-pressure valve84, for example simultaneously, define the transition from the approach stage to the actual braking stage.

As the travel of the brake pedal continues, the pressure in the first chamber A increases, enabling the desired braking to be achieved. During this stage, the interior of the chamber104is at ambient pressure, the interior of the second chamber B is at the calibration or intervention pressure of the discharge valve80(for example, 5 bar) and the first chamber A is at the pressure necessary for braking.

FIG. 5shows the positions of the parts of the cylinder during the braking stage.

The pressure difference between the first and second chambers ensures the necessary sealing of the first primary seal52, consequently isolating the first chamber A from the second chamber B.

Upon completion of the braking, the brake pedal is released and the unit comprising the first and second pistons is pushed by the return spring66as far as the rest position.

When the rest position has been reached, the initial conditions shown inFIG. 2, in which the first chamber A is in communication with the second chamber B through the radial holes46and the grooves44, and in which the second chamber B is in communication with the reservoir through the inlet duct20, are re-established. The pressure is consequently discharged in both chambers which are therefore, at ambient pressure in the rest position of the cylinder.

It can be appreciated from the foregoing that the provision of a differential cylinder as described above satisfies the above-mentioned need to achieve different travels and pressures during the different braking stages without, however, introducing excessive bulkiness or great complexities of structure or assembly.

In particular, the maintenance of a diameter of the first chamber A which is the same as that of a single-diameter cylinder means that there is no change in the force to be applied to the pedal to achieve the braking action. Moreover, the provision of a second, larger-diameter chamber B which is active during the approach stage means that the necessary travel of the pedal during the approach stage is reduced proportionally.

The provision, between the two chambers, of valve means which are separate from the seal of the fist piston48enables the valve means to be arranged for a prompt response in the transition between the two stages. The low-pressure valve is opened immediately or substantially immediately, as soon as a pressure above ambient pressure is established. Moreover the valve is particularly prompt in closing as soon as a pressure difference is created between the high-pressure and low-pressure chambers.

A novel concept was that of providing a particularly simple and compact low-pressure valve which enables the discharge valve to be housed inside one of the pistons. Unusually, the presence of the valve means between the two chambers is not subordinated to a connection between the valve means and a region which is at ambient pressure. Moreover, by virtue of the structural and functional simplicity, a person skilled in the art is not obliged to position the first valve means, which deactivate the larger-dimensioned piston, in the cylinder body, considerably complicating the structure and the processing stages. The provision for the insertion of both of the valve means in one of the pistons and in particular in the first piston48, limiting the more complex processing and assembly stages to that piston, is even more advantageous.

Moreover, the differential cylinder according to the present invention is particularly advantageous since its novel structure also enables known cylinders provided with discharge valves inside the piston to be modified easily whilst keeping the valve in its original position, avoiding complex operations in the cylinder body and large dimensions.

Furthermore, the position of the valve inside the first piston simplifies the general structure of the cylinder and does not further increase its size.

The above-mentioned valve is also particularly reliable both in opening and in closure, since this is its principal function, and eliminates sensations of a “step”, during the operation of the brake pedal, in the transition between the approach stage and the actual braking stage.

Moreover, the provision of both of the valve means inside an integrated block simplifies the production and assembly stages, ensuring correct positioning of the valve means relative to one another.

Naturally variants and/or additions may be provided for the embodiments described and illustrated above.

The low-pressure valve comprises a spring having very low stiffness, the presence of which renders the positioning of the respective sphere more reliable both during closure and during opening. However, it is possible for the low-pressure valve not to have the spring and for the sphere to be positioned during closure and during opening purely by the effect of the pressure difference between the high-pressure and low-pressure chambers (FIG. 6). In this case, grooves120may be provided in the seat which houses the valve-closure element to permit the flow of the liquid when the valve is opened.

In particular, the low-pressure valve may be arranged to be already open when the two pistons start to translate. In contrast with what is shown inFIGS. 1 to 5, both the discharge valve and the low-pressure valve may be assembled directly inside the axial hole60. A closure member is, for example, screwed, to the inner surface of the axial hole60to restrain the valve-closure element and any spring of the valve.

In contrast with what is described above, both the discharge valve80and the low-pressure valve84may be housed inside the second piston50, particularly when the shape of the two pistons is different.

The first chamber A may be formed directly in the cylinder body12, eliminating the bush30and the processing thereof and the seals for its installation in the cylinder body12. The grooves44may consequently be formed in the wall of the cavity14in the vicinity of the transition between the first and second chambers.

This further embodiment permits a saving in the material of the body12. It also retains the same ease of production of the cylinder body, and the same piston unit as was described for the previous embodiments. In addition to the foregoing, when the optimal configuration for a predetermined application has been achieved, savings can be made in parts and processing.

Finally,FIG. 7shows a possible embodiment of a detail of the cylinder in which the elements in common with the embodiments already described are indicated by the same reference numerals. The detail ofFIG. 7relates to the first piston48, to the second piston50, and to the integrated block106. The spheres, the resilient elements, and any filter inside the integrated block106are not shown since they correspond substantially to those illustrated and described for the previous embodiments.

The first piston48is intended to slide inside the bush30, if a bush is present, and the second piston50is intended to slide inside the second, larger-diameter portion of the cavity14.

According to one possible embodiment, the first piston48is formed, in a single piece, by the thrust portion48aand by the rod48b. The thrust portion48ahas a flange-shaped head having transverse dimensions substantially corresponding to the inside transverse dimensions of the bush30, if a bush is present.

A seat is also provided for housing sealing means, for example, formed by a first primary seal, not shown, restrained by a ring or by an undercut54, in similar manner to the primary seal52described above.

According to a possible embodiment, the rod48bhas, at the end remote from the thrust portion48a, a portion suitable for coupling with the second piston50.

Externally and in a direction away from the thrust portion48a, this coupling portion comprises a threaded portion, and the seat56for a sealing ring58, which is not illustrated but is similar to the sealing ring58described above.

According to a possible embodiment, the rod48bis hollow axially, that is, it has an axial hole60which extends throughout its length and is open at both ends.

According to a possible embodiment, the rod48bfurther comprises ducts61which extend through the thickness of the first piston between the axial hole60and the exterior of the first piston. According to a possible embodiment, the ducts61are inclined to a radial direction.

According to a possible embodiment, the second piston50is formed, in a single piece, by the thrust portion50aand by the rod50b. The thrust portion50acomprises a flange-shaped head having transverse dimensions substantially corresponding to the transverse dimensions of the second portion of the cavity14. Axial through-holes formed in the thrust portion50aof the second piston50are indicated70.

A second primary seal, not illustrated, but similar to the second primary seal72described above, is housed in a seat of the thrust portion50aof the second piston50, in contact with one end of each axial hole70and arranged in a manner such as to adhere to the walls of the cavity14.

According to a possible embodiment, the axial hole74having portions with different diameters for housing the coupling portion of the first piston48is formed in the head which constitutes the thrust portion50a. According to the embodiment shown inFIG. 7, there may be a first cylindrical portion for cooperating with the sealing ring of the first piston48, a threaded portion for cooperating with the corresponding threaded portion of the first piston48, and a second cylindrical portion which terminates in a base surface.

The ducts76which extend through the whole thickness of the wall of the rod50b, opening outside it, extend from the second cylindrical portion of the axial hole74.

According to a possible embodiment, the shank78, for example, a threaded shank for coupling with the control pedal, is provided at the end remote from the thrust portion50a.

According to a possible embodiment, the first piston48and the second piston50are fixed firmly to one another, as shown, for example, inFIG. 7. When the two pistons are assembled, shims79may be interposed between the two pistons so as to define their relative positions precisely. This relative position depends on the distance between the inlet duct20and the radial ducts46.

As shown inFIG. 7, when the two pistons are assembled, the axial hole60opens into the base portion of the axial hole74and is put into communication with the ducts76.

FIG. 7also shows the ring nut90which restrains and guides the two pistons, and which is substantially similar to that illustrated and described above.

The integrated block106is housed inside the first piston, in particular inside the axial hole60, and is preferably inserted from the end of the first piston48which is mounted on the second piston50. Shoulders122on the internal wall of the axial hole60constitute abutment elements for the integrated block106. Optionally, these shoulders122may be formed with inclined walls124suitable for housing a seal126.

The integrated block106comprises both the discharge valve80and the low-pressure valve84, which are housed in the cylindrical body108that can be fitted in the axial hole60. The arrangement of the two valves is similar to that shown inFIG. 2.

According to a possible embodiment, the integrated block106also comprises the filter, arranged between the two valves as shown inFIG. 2. The integrated block106also comprises radial ducts110which can be arranged in the region of or in communication with the ducts61.

According to a possible embodiment, the outer wall of the integrated block106has two seats112, disposed on opposite sides of the radial ducts110and suitable for housing seals114.

The cylindrical body108, both in the embodiment ofFIG. 7and in that ofFIG. 2, advantageously has a central portion108ahaving transverse dimensions substantially corresponding to those of the axial hole60and two end portions108bwith transverse dimensions shorter than those of the central portion. According to a possible embodiment, the end portion108bwhich is disposed at the end at which the first piston48is fixed to the second piston50projects from the first piston48. According to a possible embodiment, the base portion of the axial hole74has transverse dimensions substantially similar to those of the end portion108bso as to house and guide it.

In the embodiment ofFIG. 7, the integrated block is fitted in the first piston48and held in position by the presence of the second piston50, when the second piston50is screwed to the first.

Advantageously, there are inserted between the integrated block106and the second piston50shims79and possibly a spacer128which increases the interaction surface between the second piston50and the integrated block106. Even more advantageously, the shims79and any spacer128are of annular shape and can be fitted around the respective end portion108bof the integrated block106.

The first piston48is hollow and houses the integrated block106completely, allowing it to enter and to be secured from the end which faces the second piston50.

A further sealing ring130may be provided between the second piston50and the integrated block106or the shims79with any spacer128.

In order to satisfy contingent and specific requirements, a person skilled in the art may apply to the above-described preferred embodiment of the master cylinder many modifications, adaptations and replacements of elements with other functionally equivalent elements without, however, departing from the scope of the appended claims.