GB2247291A - Tandem vacuum booster - Google Patents

Abstract

In a tandem vacuum booster, the inner periphery of the front booster piston 4 and associated diaphragm bead 5a are clamped between the front end of a sleeve 12 fitted to the outer periphery of the piston boss 10 and a flange 10a on the boss. The inner periphery of the rear booster piston 6 and the associated diaphragm bead 7a are clamped between a stop member 13 mounted on the piston boss 10 and the rear end of the sleeve 12. This arrangement is particularly easy to assemble. Further, a first passage 14 for connecting the front and rear working chamber 2b, 3b with the control valve 32, and a second passage 15 for connecting the front and rear vacuum chambers 2a, 3a with the control valve 32 extend through both the piston boss 10 and the sleeve 12, and communication between the passages is inhibited by a single annular sealing member 16. <IMAGE>

Description

TANDEM-TYPE VACUUM BOOSTER The field of the present invention is tandem-type vacuum boosters of the type comprising a booster shell having a partition plate fixedly mounted therein for partitioning an interior of the booster shell into a front shell chamber and a rear shell chamber, a front booster piston provided to divide the front shell chamber into a fore-side front vacuum chamber and a back-side front working chamber, a front diaphragm superposed on a rear surface of the front booster piston, a rear booster piston provided to divide the rear shell chamber into a foreside rear vacuum chamber and a back-side rear working chamber, a rear diaphragm superposed on a rear surface of the rear booster piston, the front booster piston and the front diaphragm being joined to the rear booster piston and the rear diaphragm through a piston boss which extends through the partition plate and leads to an output rod, a valve cylinder continuouly formed with a rear end of the piston boss and slidably supported on a rear wall of the booster shell, an input rod for advancing and retreating movements, and a control valve for placing the two working chambers into alternate communication with the atmosphere and the two vacuum chambers in response to the advancing and retreating movements of the input rod, the input rod and the control valve being disposed within the valve cylinder.

Such a tandem type vacuum booster has already been proposed by the presentapplicant and is known, for example, from U.S. Patent No.4,905, 573.

In the above tandem type vacuum booster, in order to bind the front and rear booster pistons and the front and rear diaphragms to the piston boss, three components: the booster boss, an end plate of a cylindrical connecting member connected to the front booster piston and superposed on a front end of the piston boss, and a retaining plate superposed on a rear end of the piston boss are coupled to one another by a plurality of throughbolts, thereby fixing the front booster piston and the front diaphragm between the end plate and the front end of the piston boss and fixing the rear booster piston and the rear diaphragm between the retaining plate and the rear end of the piston boss.

In the above prior art tandem type vacuum booster, however, there is a problem that it is necessary to pass the plurality of throughbolts through the piston boss, the end plate and the retaining plate and tighten them. This results not only in an increased number of parts but also in necessitating much labor and time for assembling the parts.

Accordingly, it is an object of the present invention to provide a tandem type vacuum booster wherein the operation of coupling the front and rear booster pistons and the front and rear diaphragms to the piston boss can be readily conducted.

To achieve the above object, according to the present invention, there is provided a tandem type vacuum booster comprising a booster shell having a partition plate fixedly mounted therein for partitioning an interior of the booster shell into a front shell chamber and a rear shell chamber; a front booster piston provided to divide the front shell chamber into a fore-side front vacuum chamber and a back-side front working chamber; a front diaphragm superposed on a rear surface of the front booster piston; a rear booster piston provided to divide the rear shell chamber into a fore-side rear vacuum chamber and a back-side rear working chamber; a rear diaphragm superposed on a rear surface of the rear booster piston; the front booster piston and the front diaphragm being Joined to the rear booster piston and the rear diaphragm through a piston boss which extends through the partition plate and leads to an output rod; a valve cylinder continuously formed with a rear end of the piston boss and slidably supported on a rear wall of the booster shell; an input rod for advancing and retreating movements; and a control valve for placing both the working chambers into alternate communication with the atmosphere and both the vacuum chambers in response to advancing and retreating movements of the input rod, the input rod and the control valve being disposed within the valve cylinder, wherein the booster further includes a sleeve slidably supported on the partition plate and fitted to an outer periphery of the piston boss, an inner peripheral end of the front booster piston and an inner peripheral bead of the front diaphragm being clamped between a front end of the sleeve and a flange provided on the piston boss and being thereby fixed to a front portion of the piston boss, and an inner peripheral end of the rear booster piston and an inner peripheral bead of the rear diaphragm being clamped between a stop member mounted on the piston boss and a rear end of the sleeve and being thereby fixed to a rear portion of the piston boss.

With the above construction, only the two members, namely the sleeve fitted over the piston boss and the stopper member mounted on the piston boss, are needed to fix the inner peripheral edges of the front and rear booster pistons and the inner peripheral beads of the front and rear diaphragms to the piston boss, and it is therefore possible to reduce the number of parts and substantially to shorten the time required for assembling them, as compared with the prior art using a plurality of throughbolts.

According to a development of the present invention, there is provided a tandem-type vacuum booster further including a first port for communicating the front and rear working chambers with the control valve, a second port for communicating the front and rear vacuum chambers with the control valve, the first and second ports being formed to extend between both the piston boss and the sleeve, and a single annular sealing member mounted in an axially intermediate portion of the junction surface between the piston boss and the sleeve, the first and second ports passing through the junction surface, in a generally radial direction, to the front and to the rear of the annular sealing member respectively.

With this construction, not only can each of the ports be made easily by utilising the junction surface between the piston boss and the sleeve, but also communication between both ports is reliably inhibited by means of the single annular sealing member.

Further, according to a third aspect of the present invention, there is provided a tandem type vacuum booster wherein a port is provided for communicating the front and rear vacuum chambers with the control valve, and the port is comprised of a first passage axially extended through the piston boss to communicate with the front vacuum chamber, and a second passage diverged radially outwardly from an intermediate portion of the first passage to communicate with the rear vacuum chamber, the second passage having an opening which is extended along a front surface of the rear booster piston and opens to the rear vacuum chamber With this construction, it is possible, in the advancing movement of the piston boss on the partition plate, to insure a sufficiently large advancing stroke of the piston boss until the opening of the second passage communicating with the rear vacuum chamber reaches a location corresponding to the partition plate. Consequently, it is possible to shorten the axial dimension of the rear vacuum chamber to a minimal level substantially equal to the stroke of the piston boss to contribute to a reduction in size of the tandem type vacuum booster.

The above and other objects, features and advantages of the invention will become apparent from a reading of the following description of the preferred embodiment, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig.lis a longitudinal sectional view illustrating a tandem type vacuum booster out of operation; Figs.2 and 3 are sectional views taken along lines II-II and III III in Fig.1, respectively; and Fig.4 is a longitudinal sectional view illustrating a part of the tandem-type vacuum booster in operation.

The present invention will now be described by way of one embodiment in connection with the accompanying drawings.

Referring to Fig.1, a brake master cylinder M which is operated by a tandem type vacuum booster B is attached to a front surface of a booster shell 1 of the vacuum booster B.

The booster shell 1 is comprised of a pair of front and rear shell halves la and Ib coupled at opposed ends thereof to each other, and a partition plate 1c clamped between both the shell halves la and ib to partition the inside of the booster shell 1 into a front shell chamber 2 and a rear shell chamber 3. The rear shell half ib is supported on a vehicle body which is not shown.

The front shell chamber 2 is divided into a fore-side front vacuum chamber 2a and a back-side front working chamber 2b by a front booster piston 4 received in the front shell chamber 2 for longitudinally reciprocal movement and by a front diaphragm 5 superposed on and coupled to a rear surface of the front booster piston 4 and clamped between the front shell half la and the partition plate Ic. The rear shell chamber 3 is divided into a fore-side rear vacuum chamber 3a and a back-side rear working chamber 3b by a rear booster piston 6 received in the rear shell chamber 3 for longitudinally reciprocal movement and by a rear diaphragm 7 superposed on and coupled to a rear surface of the rear booster piston 6 and secured together with the partition plate ic between the shell halves la and lb.

The front and rear booster pistons 4 and 6 formed annularly from a steel sheet are coupled to front and rear ends of a piston boss 10 made of synthetic resin, respectively, and a sleeve 12 is fitted over an outer periphery of the piston boss 10 to lie between the booster pistons 4 and 6 and is slidably carried on the partition plate Ic with a bush 8 and a sealing member 9 interposed therebetween.

More specifically, the piston boss 10 is provided at its front end face with a circular recess 11 having a depth amounting to substantially half of the length of the boss 10, and with a flange 10a projecting at a peripheral edge of an opening of the circular recess 11. An inner peripheral end of the front booster piston 4 and an inner peripheral bead 5a of the front diaphragm 5 are engaged, in a superposed relation, with the flange iota, and the cylindrical sleeve 12 is placed over an outer periphery of the piston boss 10 forwardly from the rear, so that a front end thereof abuts against a rear surface of the inner peripheral bead sa. This causes the front booster piston 4 and the front diaphragm 5 to be clamped between the flange 10a and the sleeve 12 and thereby secured firmly to a front portion of the piston boss 10.An inner peripheral end of the rear booster piston 6 and an inner peripheral bead 7a of the rear diaphragm 7 are engaged, in a superposed relation, with a step lOb formed in the piston boss 10 inside a rear end of the sleeve 12, and the rear surface of the inner peripheral bead 7a is retained by a set ring 13 as a stop member. More specifically, the set ring 13 is mounted over the piston boss 10 by engagement of a large number of pawls 13a projecting on the inner periphery of the set ring 13 into an annular groove lOc in the rear portion of the step lOb, so that the rear booster piston 6 and the rear diaphragm 7 are secured firmly to the rear portion of the piston boss 10 by the set ring 13, and axial movement of the sleeve 12 fitting over the piston boss 10 is restrained by the set ring 13.

A first port 14 is formed in the piston boss 10 to communicate the front and rear working chambers 2b and 3b with the atmosphere through a control valve 32 which will be described hereinafter. As can be seen by reference to Figs.2 and 3 in addition to Fig.l, the first port 14 includes a passage 14a extending radially through the rear of the piston boss 10 to permit communication between the rear working chamber 3b and the control valve 32, a passage 14b having an arcuate section and made axially in the piston boss 10, a passage 14c extending radially outwardly from a front end of the passage 14b, and a passage 14d defined by a groove provided in the inner surface of the sleeve 12 and by the outer periphery of the piston boss 10 to communicate with the back-side front working chamber 2b.

A second port 15 is provided for communicating the front and rear working chambers 2b and 3b with the front and rear vacuum chambers 2a and 3a through the first port 14 and the control valve 32 and consists of a passage 15a having an arcuate section and passing axially through the piston boss 10 to connect the control valve 32 with the front vacuum chamber 2a, a passage 15b extending radially outwardly from the passage 15a, and a passage 15c defined by a groove provided in the inner surface and rear end of the sleeve 12 with the front surface of the rear booster piston 6 to permit communication between the passage 15b and the rear vacuum chamber 3a.

A recess lOd provided in the outer periphery of the piston boss 10 and a projection 12a formed on the inner periphery of the sleeve 12 are engaged with each other (see Fig.3) in order to provide an alignment of the passages 14c, 15b provided in the piston boss 10 and the passages 14d, 15c provided in the sleeve 12. Contact surfaces of the outer periphery of the piston boss 10 and the inner periphery of the sleeve 12 are sealed by an O-ring 16 mounted in a seal groove 10e provided in the outer periphery of the piston boss 10. Thus, the first port 14 communicating with the front working chamber 2b is prevented from communicating with the front and rear vacuum chambers 2a and 3a by the inner peripheral bead 5a of the front diaphragm 5 and the O-ring 16, and the second port 15 communicating with the both the vacuum chambers 2a and 3a is prevented from communicating with the rear and front working chambers 3b and 2b by the inner peripheral bead 7a of the rear diaphragm 7 and the O-ring 16.

A return spring 23 is mounted in a compressed manner between the circular recess 11 provided at the front end of the piston boss 10 and the front shell half la, so that the piston boss 10 and thus both the booster pistons 4 and 6 are biased in retreating directions at all times by a spring force of the return spring 23. A retreat limit for the booster pistons 4 and 6 is determined by abutment of a large number of projections 24 raised on the rear surface of the rear diaphragm 6 against a rear wall of the booster shell 1.

The front vacuum chamber 2a is connected to a vacuum pressure source (not shown) (e.g., an inside of an intake manifold in an internal combustion engine) through a vacuum pressure intake pipe 29 and communicates with the rear vacuum chamber 3a via the second port 15 made in the piston boss 10. The front and rear working chambers 2b and 3b communicate with each other via the first port 14 made in the piston boss 10 and is adapted to be put in an alternately switched manner into communication with the front and rear vacuum chambers 2a and 3a and an atmospheric air intake port 33 opened into an end wall 26a of a rearwardly extended cylindrical member 26 connected to the rear end of the booster shell 1.

An input rod 35 connected to a brake pedal 34 and the control valve 32 controlled by the input rod 35 are mounted in the following manner within a valve cylinder 25 connected to the rear portion of the piston boss 10. A valve piston 38 is slidably received in a front portion of the valve cylinder 25, and the input rod 35 extending through the atmospheric air intake port 33 is ocillatably coupled at its front end to the valve piston 38. A first annular valve seat 39 is provided on an inner peripheral surface of the valve cylinder 25, and a second annular valve seat 40 is formed on a rear end face of the valve piston 38 and surrounded by the first valve seat 39. A valve member 41 is disposed within the valve cylinder 25 to cooperate with the valve seat 39 and 40.

The valve member 41 is made of rubber and is cylindrical with opposite front and rear ends opened. The rear or base end 41a of the valve member 41 is kept in close contact with the inner peripheral surface of the valve cylinder 25 by a cylindrical retainer 42 fitted to the inner peripheral surface of the valve cylinder 25. The valve member 41 includes a flexible portion 41b having a smaller wall thickness and bent radially inwardly from the base end 41a, and a valve portion 41c having a larger wall thickness and connected to a front end of the flexible portion 41b, the valve portion 41c being disposed in an opposed relation to the first and second valve seats 39 and 40.The valve portion 41c is movable longitudinally by deformation of the flexible portion 41b, so that it is seated on the first and second valve seats 39 and 40 upon advancing movement and received on a front end of the cylindrical retainer 42 upon retreating movement.

An annular reinforcing plate 43 is embedded in the valve portion 41c, and a valve spring 44 is mounted in a compressed manner between the reinforcing plate 43 and the input rod 35 for biasing the valve portion 41c toward both the valve seats 39 and 40. One end of the second port 15 is opened at the inner surface of the valve cylinder 25 outside the first valve seat 39, and one end of the first port 14 is opened at the inner surface of the valve cylinder 25 inside the first valve seat 39.

A space inside the second seat 40 communicates with the atmospheric air intake port 33 via the hollow portions of the valve member 41 and the cylindrical retainer 42. Thus, the control valve 32 is comprised of the valve member 41, the valve spring 44, and the first and second valve seats 39 and 40.

A return spring 45 is mounted in a compressed manner between the input rod 35 and the cylindrical retainer 42 for biasing the input rod 35 toward its retreat limit. The retreat limit of the input rod 35 is provided by abutment of a stopper plate 46 screwed over the input rod 35 for advancing and retreating movements against the inner surface of the rear end 26a of the cylindrical rearwardly-extended member 26 through a washer 48. Therefore, if the stopper 46 is turned, the screwed position of the stopper plate 46 on the input rod 35 is changed and hence, the retreat limit of the input rod 35 can be adjusted longitudinally. The fixing of the stopper plate 46 after such adjustment may be achieved by tightening of a lock nut 47 screwed on the input rod 35.A communication hole 48a is provided in the washer 48, so that the atmospheric air intake port 33 is prevented from being occluded by the washer 48.

An air filter 49 is mounted to surround the input rod 35 for filtering air introduced through the atmospheric air intake port 33 into the valve cylinder 25. The air filter 49 has a moderate softness, so that the relative displacement between the input rod 35 and the valve cylinder 25 is not'hindered.

The piston boss 10 is provided with a larger diameter cylinder bore 37 opened at a central portion of the front surface thereof, and a smaller diameter cylinder bore 36 opened at its opposite ends into the larger diameter cylinder bore 37 and the valve cylinder 25. A reaction piston 52, which may be provided either integrally with or to abut against the valve piston 38, is slidably received in the smaller diameter cylinder bore 36, and a resilient piston 50 and an output piston 51 superposed on a front surface of the resilient piston 50 are slidably received in the larger diameter cylinder bore 37. In order to prevent the output piston 51 from being slipped out of the larger diameter cylinder bore 37, a set ring 54 is mounted in an opening of the larger diameter cylinder bore 37.

An output rod 53 is provided projectingly on a front surface of the output piston 51 and connected to a piston 55 in the brake master cylinder M.

The operation of this embodiment will be described below. First, when the vacuum booster B is out of operation, the input rod 35 is located at the retreat limit, and the control valve 32 is in a neutral state in which the valve portion 41c is seated on the first and second valve seats 39 and 40 to put both the front and rear working chambers 2b and 3b out of communication with either the vacuum chambers 2a and 3a and the atmospheric air intake port 33, as shown in Fig.l. The control valve 32 in such a state ensures that a vacuum pressure supplied from the vacuum pressure source through the vacuum pressure intake pipe 29 is'stored in both the vacuum chambers 2a and 3a, and a vacuum pressure diluted properly with the atmospheric air is maintained in both the working chambers 2b and 3b. Thus. a slight advancing force is applied to each of the front and rear booster pistons 4 and 6 due to a difference in air pressure developed between the front vacuum chamber 2a and the working chamber 2b and between the rear vacuum chamber 3a and the working chamber 3b, but such advancing forces are balanced with the resilient force of the return spring 23, so that the booster pistons 4 and 6 are stopped at locations to which they are advanced slightly from their retreat limits.

If the brake pedal 34 is now depressed to advance the input rod 35 and the piston 38 in order to brake the vehicle, the second valve seat 40 is immediately separated away from the valve portion 41c to put both the working chambers 2b and 3b into communication with the atmospheric air intake port 33, because both the booster pistons 4 and 6 are stationary at the beginning. As a result, the atmospheric air is introduced quickly through the atmospheric air intake port 33 via the second valve seat 40 and the first port 14 into both the working chambers 2b and 3b, so that the pressures in the chambers 2b and 3b are brought into a level higher than that in the vacuum chambers 2a and 3a.

Therefore, as shown in Fig.4, the booster pistons 4 and 6 get a larger advancing force based on a difference between the air pressures to advance with a good response against the force of the return spring 23, thereby driving the piston 55 of the brake master cylinder n forwardly through the output rod 53. In this manner, the brake master cylinder M can be operated without any retard from the depression of the brake pedal 34 to brake the vehicle. During this time, the passage 15c of the second port 15 made in the piston boss 10 is opened at the rear end of the rear vacuum chamber 3a and hence it is possible to insure a sufficient stroke of the piston boss 10 and thus the piston 55.

During such a braking, the valve piston 38 is also advanced together with the input rod 35 to abut against the resilient piston 50 with the reaction piston 52 interposed therebetween. However, the resilient piston 50 is deformed to expand toward the smaller diameter cylinder bore 36 upon reception of a reaction force from the operation of the booster pistons 4 and 6, thereby applying a portion of such reaction force to the reaction piston 52, so that this force is fed back to the brake pedal 34 through the valve piston 38 and the input rod 35.

Such a reaction effect enables the driver to sense the magnitude of an output from the output rod 53, i.e., a braking force.

If the output from the output rod 53 exceeds a boosting limit point due to an increase in depression force to the brake pedal 34, i.e., in input to the input rod 35, a front surface of the valve piston 38 is abutted against the piston boss 10, so that the entire input is transmitted via the valve piston 38, the piston boss 10, the resilient piston 50 and the output piston 51 to the output rod 53, and ultimately, a sum of the advancing force for each of the booster pistons 4 and 6 due to a difference in air pressure and the advancing force due to the input is output from the output rod 53.

If the depression force to the brake pedal 34 is then released, the input rod 35 is first retreated along with the valve piston 38 by the resilient force of the return spring 45, thereby allowing the second valve seat 40 to be seated on the valve portion 41c of the valve member 41, while allowing the valve portion 41c to be separated greatly away from the first valve seat 39, so that both the working chambers 2b and 3b are put into communication with the vacuum chambers 2a and 3a to immediately eliminate the difference in air pressure across each of the booster piston 4 and 6. Thus, the booster pistons 4 and 6 are retreated by the resili ent force of the return spring 23 to release the operation of the brake master cylinder M.If the input rod 35 is returned to its retreat limit at which the stopper plate 46 abuts against the end wall 26a of the cyli ndrical extension 26, the rear booster piston .6 is once returned to its retreat limit at which the projections 24 abut against the rear wall of the booster shell 1, thereby allowing the first valve seat 39 to be seated on the valve portion 41c, while allowing the valve portion 41c to be separated slightly away from the second valve seat 40, so that the atmospheric air is introduced again into both the working chambers 2b and 3b. If both the booster pistons 4 and 6 are advanced slightly due to a difference in air pressure developed thereby, however, a small gap between the second valve seat 40 and the valve portion 41c is eliminated to bring the control valve 32 into the initial neutral state.In this manner, the vacuum pressure diluted with the atmospheric air is maintained in both the working chambers 2b and 3b, and the vacuum booster B is brought into an inoperative state as shown in Fig. 1.

In suchatandem type vacuum booster B, in order to fix the front and rear booster pistons 4 and 6 and the front and rear diaphragms 5 and 7 to the piston boss 10, the inner peripheral end of the front booster piston 4 and the inner peripheral bead 5a of the front diaphragm 5 are first fitted to the flange lOa of the piston boss 10 from the rear, and then the front booster piston 4 and the diaphragm 5 are clamped and fixed between the flange l0a and the front end of the sleeve 12 placed from the rear. At this time, relative rotation between the sleeve 12 and the piston boss 10 is restrained by engagement of the projection 12a formed on the sleeve 12 into the recess 10d made in the piston boss 10.In addition, the sealing member 9 mounted on the partition plate ic is bent forwardly, i.e., toward the inside of the front working chamber 2b by the front end of the sleeve 12 placed from the rearward, thus effectively preventing any leakage of pressure from the higher pressure front working chamber 2b into the lower pressure rear vacuum chamber 3a. Then the inner peripheral end of the rear booster piston 6 and the inner peripheral bead 7a of the rear diaphragm 7 are set to the step lOb of the piston boss 10 from the rear, and the set ring 13 is then mounted onthe rear portion thereof. This causes the rear booster piston 6 and the rear diaphragm 7 to be clamped and fixed between the rear end of the sleeve 12 and the set ring 13.

With the sleeve 12 fixed to the piston boss 10 in the above manner, the first port 14 is defined by cooperation of the piston boss 10 and the sleeve 12, and the second port 15 is defined by cooperation of the piston boss 10, the sleeve 12 and the rear booster piston 6. The communication of the first port 14 with the front and rear vacuum chambers 2a and 3a is inhibited by the O-ring 16 disposed between the outer periphery of the piston boss 10 and the inner periphery of the sleeve 12 and by the inner peripheral bead 5a of the front diaphragm 5, and at the same time the communication of the second port 15 with the front and rear working chambers 2h and 3b is inhibited by the O-ring 16 and the inner peripheral bead 7a of the rear diaphragm 7.

Claims (4)

CLAIMS:

1. A tandem-type vacuum booster comprising a booster shell having a partition plate fixedly mounted therein for partitioning the interior of the booster shell into a front shell chamber and a rear shell chamber; a front booster piston provided to divide the front shell chamber into a front vacuum chamber and a front working chamber; a front diaphragm superposed on the rear surface of the front booster piston; a rear booster piston provided to divide the rear shell chamber into a rear vacuum chamber and a rear working chamber; a rear diaphragm superposed on the rear surface of the rear booster piston; a piston boss extending through the partition plate and leading to an output rod, the piston rod joining the front booster piston and front diaphragm to the rear booster piston and rear diaphragm;; a valve cylinder at the rear end of the piston boss and slidably supported in the rear of the booster shell; an input rod for advancing and retreating movements; and a control valve for placing both working chambers alternately into communication with the atmosphere and with the vacuum chambers in response to advancing and retreating movements of the input rod, the input rod and control valve being disposed within the valve cylinder;; wherein the booster further includes a sleeve slidably supported on the partition plate and fitted to the outer periphery of the piston boss and a stop member mounted towards the rear of the piston boss, the inner periphery of the front booster piston and of the front diaphragm being clamped between the front end of the sleeve and a flange provided on the piston boss and being thereby fixed to the front portion of the piston boss, and the inner periphery of the rear booster piston and of the rear diaphragm being clamped between the stop member and the rear end of the sleeve and being thereby fixed to the rear portion of the piston boss.

2. A tandem-type vacuum booster according to claim 1, further including a first port for connecting the front and rear working chambers with the control valve, a second port for communicating the front and rear vacuum chambers with the control valve, these first and second ports being formed to extend between both the piston boss and the sleeve, and a single annular sealing member mounted in an axially intermediate portion of the junction surface between the piston boss and the sleeve, the first and second ports passing through the junction surface to the front and rear of the annular sealing member, respectively.

3. A tandem-type vacuum booster according to claim 1, wherein a port is provided for communicating said front and rear vacuum chambers with the control valve, this port comprising a first passage extending generally axially through the piston boss to communicate with the front vacuum chamber, and a second passage diverging generally outwardly from an intermediate portion of the first passage to communicate with the rear vacuum chamber, the second passage having an opening which extends along the front surface of the rear booster piston and opens to the rear vacuum chamber.

4. A tandem-type vacuum booster substantially as described herein with reference to the accompanying drawings.