Abstract: A compressed air pump comprising a compressed air piston motor (11) having a cylinder housing (12) and a low pressure piston (22) moveable between first and second end wall parts (13, 14) and a hydraulic piston pump (15) in the second end wall part (14). One side of the low pressure piston and the first end wall part together define a working chamber (21) and the other side of the piston (22) abuts and drives the hydraulic piston (24). A pilot piston (25), which is controlled by the low pressure piston (22) controls the supply of compressed air to the working chamber (21). A spring powers the return stroke of the low pressure piston and the hydraulic piston. The pilot piston (25) is housed in a pilot cylinder (35) incorporated in the first end wall part (13) and is mounted for movement between a first position and a second position. In the first position compressed air can be passed from an inlet (17) in the end wall part to the working chamber.