A vacuum motor operatively connected to a slave cylinder operates same via pressure differential variations across a reciprocatable vacuum motor piston to pump working fluid from a reservoir to an accumulator through two one way valves. The pressure differential variation can be derived from gear changing of the vehicle, induced through a three way electromagnetic valve controlled by a circuit characterized by either an unstable multivibrator, a dual stable multivibrator, a thyristor or a self maintaining relay, or induced through a purely mechanically operated valve characterized by lost motion and snap action position maintain devices. A pressure relief valve may be provided which dumps excessive pressurized fluid in the accumulator and returns same to the reservoir.