GB939335A - Vehicle brake master cylinder - Google Patents

Abstract

939,335. Master cylinders. KELSEY-HAYES CO. Jan. 24, 1962, No. 2600/62. Class 103 (1). In a master cylinder of the type in which first and second pistons are provided in a common bore, the first piston being moved directly by an applying member and the second piston floating between the first piston and the remote end of the cylinder so as to divide the bore into two chambers connected to different parts of the braking system, the two pistons are connected together for limited relative movement towards and away from one another, and are biased towards positions in which the first piston is retracted and the second piston is at its maximum spacing from the first piston, in which positions a valve member arranged to move coaxially with the pistons and connected with the second piston is held off a seat in the remote end of the cylinder so as to communicate the chamber remote from the first piston to a reservoir. In one embodiment, Fig. 1, the master cylinder comprises a reservoir 11 divided below a certain level by a partition 12 into two compartments 13, 14, a bore 15 containing first and second pistons 16, 24, respectively, a port 22 connecting compartment 13 with bore 15 behind a cup washer 17 on the first piston, a compensating port 23 connecting compartment 13 with bore 15 in front of the cup washer 17, a passage 40 connecting compartment 14 with a valve chamber 41, and ports 52, 53, respectively connecting chambers 26, 27 on either side of piston 24 via residual pressure valves (Fig. 3, not shown) to front and rear brake cylinders 61, 63, respectively. A stem 36 of piston 24 extends into a recess 21 in the piston 16 and engages behind a cup washer 30 which is held against the front end of piston 16 by a spring 31 engaging a snap-ring 33 seated against a shoulder 34 in the bore 15. A second spring 32 extends between the cup washer 30 and the head 25 of piston 24 to bias the pistons apart. A valve member 44 adapted to engage a seat 42 at the entrance to chamber 41 is normally retained clear of the seat by a head 45 engaging a disc 48 secured within a recess 46 in the piston 24. A spring 47 engages behind the head 45 to urge the valve member in the closing direction. In operation, the spring 31 biases both pistons to retracted position and spring 32 holds the pistons at maximum spacing, in which positions valve member 44 is held off the seat 42. When piston 16 is moved, either manually or via a servo, through a rod 19, the pistons initially move together, causing valve member 44 to close, after which pressure builds up in chamber 27 to pressurize the rear brakes. Pressure does not build up in the front brakes, however, until the applying force is sufficient to overcome the resistance of prestressed spring 32, so that differential braking is obtained. Should a leak occur in the rear brakes piston 24 abuts the end wall 49 so that pressure is maintained in chamber 26. Should a leak occur in the front brakes piston 16 engages the back of piston 24 after stem 36 has moved into the recess 21, so that pressure is maintained in chamber 27. In a second embodiment, Fig. 4, the first and second pistons 70, 72 are biased rearwardly by separate springs 85, 96, the former being arranged between the two pistons and the latter between the second piston and a plate 94 seated on the end wall of the cylinder. The retracted position of the second piston is determined by a stem 89 of a valve member 88 which is held off the seat 42 by the spring 96. A spring 95 applies the valve member 88 to the seat 42 as soon as piston 78 is moved forwardly. Spring 96 is stronger than spring 85 so that on movement of rod 19 forwardly chamber 80, which is connected to the rear brakes via port 81, is pressurized first, chamber 83 being subsequently pressurized to pressurize the front brakes when the compression of spring 96 has been overcome.