The pump has a body including at least a fuel compression chamber and an actuating chamber enclosing the actuating members of the pump. The on-off valve has a shutter sliding inside a hole in the body to close a fuel feed conduit. The shutter is held in the closed position by a compression spring, which rests directly or indirectly on a shoulder in the hole. In one embodiment, the spring rests on the shoulder via a perforated disk held by a retaining ring, which clicks removably inside an annular groove in the hole. In a further embodiment, the spring rests directly on the shoulder.