Found under the following brands:
Annovi Reverberi-AR, Westward, Bostitch, Black & Decker, Craftsman, Devilbiss Air Power, DeWalt, Emglo, Excell, Porter Cable, Speedaire
Order if valve is broken. Also order head gasket and cylinder gasket if torn. Poor performance or air leaks out of the intake filter while the unit is running? This is probably the problem.