Abstract: A pilot spool is interposed between the compressed air supply and the diaphragm pump-operating main spool, to insure that, at any one time, only one end of the bore in which the main spool reciprocates receives the compressed air. The pilot spool has an annular recess of a length which does not bridge across passageways which admit the air to the main spool, so alternatively, each passageway opens onto an end of the main spool bore. The reciprocating pump structure, that is, the double diaphragms, shaft, and clamp plates, cause the pilot spool to shift, to communicate one of the passageways, with each shift, with the air supply and the main spool bore. No lubrication is provided or needed, and the pilot spool insures that the main spool will not stall.