to accomodate the stepped external face of the sleeve 30 so that an annular cavity 34 is formed the annular chamber 34 connects with the outside through a connection 36 and encloses a coiled compressed spring 35 normally urging the sleeve 30 to the plug 14 the sleeve 30 encloses the rod 13 with a clearance therebetween so that passage of fluid is permitted therethrough the chamber 16 is communicated through a conduit 37 and conduit 18 in the upper plug 17 with the chamber 15 the bottom end of the rod 13 is in resiliently abutment relation with a load sensing leaf spring or resilient member 38 rotatably attached at one end 39 to the device on the chassis of the vehicle i e a first mass and at the other end 40 to the axle housing i e a second mass also rotatably through a lever member pivoted to the axle housing so that the piston 12 is upward spring biased upon flow of pressure fluid from the rear master cylinder to the connection 25 fluid flows through the chamber 16 thence through conduit 37 conduit 18 valve 21 to the chamber 15 when the pressure of the fluid from the master cylinder reaches a value such that the total hydraulic thrust on the piston 12 acting downward as viewed in fig1 is higher than the total thrust on the piston by spring 26 and load sensing spring 38 the downward stroke of the piston 12 begins when the.