1,014,963. Hydraulic braking systems of vehicles. ROCKWELL-STANDARD CORPORATION. Sept. 30, 1963 [Nov. 30, 1962], No. 38346/63. Headings F2E and F2F. In a hydraulic braking system of a multiwheeled vehicle of the kind having primary and secondary hydraulic systems, pressure being created in the primary system by a pedal-operated master cylinder to apply the brakes on one or more wheels whereby drag forces acting on the shoes generate pressure in the secondary system to apply the brakes on the other wheels, the two systems are connected at the output port of the pedal-operated master cylinder and a valve responsive to pressure in the primary system opens to interconnect the systems only when no pressure exists in the primary system. As shown, front wheel brake assemblies 18, 19 comprise slave cylinders 16, 17 operated directly by pedal-operated master cylinder 11 to apply adjacent ends of shoes 21, 22 to brake drums of the respective assemblies, the opposite ends of the shoes in each case being connected as a servo-shoe assembly by a floating adjuster 23 so that the drag forces tend to rotate the shoe assemblies, so producing pressure in pressure generating cylinders 55, 56 formed integrally with the cylinders 16, 17. The cylinders 55, 56 have pistons 58, 59 urged apart by a spring 61 and retained against outward movement from the rest position by snap rings 62, 63. Projections 68, 69 on the shoes engage the pistons 58, 59 directly to transmit drag force to the appropriate one of the pistons and thereby generate pressure in the space 57 forming part of the secondary system for actuating slave cylinders 38, 39, 43, 44 of the rear wheel brake assemblies 36, 37. Abutments 60, 60<SP>1</SP> limit inward movement of pistons 58, 59. Cylinders 16, 17 are conventional, comprising pistons 28, 29 connected to the shoes by push rods 32, 33. The valve responsive to pressure in the primary system comprises a casing 206, 208, Fig. 10, secured via a sleeve 84 to the forward end of the master cylinder 11 and containing a piston 220 which is bored to receive a loosely fitting stem 226 of a valve plunger 228 having seat surfaces 232, 234 adapted to seat on the piston and forward end of the casing respectively, the latter closing an outlet 216 to the secondary system. When the parts are at rest spring 238 maintains the piston in engagement with closure member 208 of the casing and a spring 240 in turn retracts the stem 226 until a snap ring 242 engages the member 208, whereby fluid can pass freely between the two systems. When the master cylinder 11 is operated piston 220 is moved forwardly first to engage seat surface 232 of the plunger 228, as shown, then after further movement together with plunger 228 to initially pressurize the secondary system it moves seat surface 234 into position to close the outlet 216 so that the secondary system is cut off from the primary system. In a modification, Fig. 7, each front wheel brake assembly has a slave cylinder 91 between one pair of ends of the shoes 96, 97, operated by the master cylinder 11, and a pressure generating cylinder 100 between the opposite ends of the shoes containing pistons 107, 108 to pressurize the secondary system in response to the drag force in one or other direction of rotation. The operation is similar to that of the brake of Fig. 3 excepting that the shoes act as two leading in both directions of rotation instead of as a servo-shoe assembly, since the pistons 107, 108 are prevented from moving outwards beyond the rest position by snap rings 111, 112. In Fig. 8, the lower cylinder is modified to provide for servo-shoe operation by forming the pistons of inner and outer parts 128, 132 which are relatively slidable, limited by shoulders 135 on the parts 128. As a result, if the left piston is forced inwards, a portion of the drag force is transferred to the right shoe via the reduced area of the right piston portion 128, the remainder of the drag force being used to pressurize the secondary system. In the modification of Fig. 9 the slave and pressure generating cylinders are incorporated in one unit between the upper ends of the shoes, the slave cylinder 161 having a closed end 162 engaging shoe 164 and an open end engaging annular piston 147 of pressure generating cylinder 141, a second annular piston 146 in cylinder 141 being bored to receive piston 154 which operates in the slave cylinder 161 and which has an enlarged head 155 abutting both the piston 146 and shoe 158. On application of fluid to space 167 by the master cylinder 11 the shoes are forced apart and pistons 146, 147 remain at rest against snap rings 149, 151 until the drag force acts on one or other of these pistons to compress spring 148 and pressurise chamber 143 which is connected to the secondary system.