Patent GB-1381407-A (GB)
Title: Master cylinder assemblies for vehicle hydraulic braking systems
IPC: B60T11/21
Filed: 1972-07-07
Abstract: 1381407 Master cylinder assemblies GIRLINGLtd 25 June 1973 [7 July 1972] 31847/72 Heading F2F A master cylinder assembly for a vehicle hydraulic braking system comprises two master cylinders having respective transfer chambers interconnected by a transfer port or passage 15 but isolatable from the respective pressure spaces of the cylinders by respective transfer valves, e.g. 28, 29 which are normally closed but which are adapted to be opened by axial movement of piston 7 in a brake applying direction wherein at least one of the master cylinders incorporates pressure relief means defining, at least when the transfer valve 28, 29 is closed, a leak path between the transfer chamber of that master cylinder and a reservoir 4 to compensate for increase in pressure in the transfer chambers due to thermal expansion of the hydraulic fluid. Such a master cylinder assembly may be used on an agricultural tractor with the master cylinders being operated simultaneously for retarding the vehicle or separately for steering. Thermal expansion of the fluid may occur when the tractor is stationary with the prime mover running. In the embodiment of Fig. 1, the two master cylinders have an integral reservoir 4 and each master cylinder incorporates a stepped rod 26 which carries a head 23 of a recuperation valve 23, 24 and also carries a head 28 of transfer valve 28, 29. Valve seating 29 is formed on a plug 18 which is screwed into a passage 12 in the piston 7 with the interposition of an annular washer 20. The annular washer 20 is constructed from a permeable or porous material, conveniently sintered metal, although in a modification the plug 18 is constructed from this material and the washer 20 is omitted. The transfer chamber is defined by the axial passage 12 and by an annular recess 14 in the peripheral surface of the piston 7, the passage 12 and the recess 14 being interconnected by radial ports 13. In the event of the pressure of hydraulic fluid within a transfer chamber increasing due to thermal expansion, the pressure is relieved to the reservoir 4 by filtering through the material of the washer 20 or the plug 18. When one or both of the master cylinders are operated the piston 7 is moved, valve 23, 24 closes and valve 28, 29 opens, and fluid pressurized in the pressure space in front of the piston 7 is supplied to the brakes through a radial outlet port. In the modified construction of Fig. 2, the head 28 co-operates with a seating 29 defined by a shoulder 34 on piston 7. The rod 26 has a passage 35 with an enlarged port 36 containing a ball 39, and when the pressure within the transfer chamber increases fluid can leak into the reservoir via a path defined by a clearance defined between the ball 39 and the passage port 36. The ball 39 may move axially a small amount relative to a shoulder defining a seating (40, Fig. 3, not shown) and in the modification of Fig. 4 (not shown) the ball (39) engages with a seating (41) defined by an inclined step between the passage (35) and a counterbore (42). In the embodiment of Figs. 5, 6 showing one master cylinder with a transfer port 43, the ball 49 is retained in a valve chamber 46 defined in the rod 26 by means of a centrally apertured and axially slotted end cap 50 which fits over rod 26. The ball 49 is normally urged against a flange 51 of the cap 50 by a light compression spring 52. Fluid can leak into the reservoir through a path defined by the central aperture and slots in the cap 50 and a clearance between the ball 49 and the wall of valve chamber 46. The ball 49 engages a seating (54) when the other master cylinder is operated.