Abstract: The valve has a cylindrical, cartridge-like housing in which a piston is slidably disposed. The piston subdivides the housing into an outer chamber, at one housing end, into which oil under pressure is admitted, and an inner chamber from which oil is discharged. Ports formed in the housing open onto the inner chamber and allow the oil to pass therethrough and on into an associated air compressor. The other end of the valve housing defines a control chamber. The piston has a depending, cylindrical shank which (a) moves into, and removes from, the control chamber, and (b) has a disc, fixed onto the end of the shank, which is sealingly engaged with the wall of the control chamber. Channels are formed in the periphery of the piston, to allow oil to flow from the outer chamber to the inner chamber, but a land in the housing defines a seat for the piston which, when the piston closes thereupon, interdicts the aforesaid oil flow.