[whited44]

Just rebuilt Gast compressor with the rebuild kit from Gast (about $241).  Easy to do, but not for sure if needed.  The biggest problem was the compressor outflow check valve allowing air to feed back into compressor.  I installed a new pressure relief valve that is on the opposite head and it is no better than the original.  During my test (test jig into a big portable tank, about 10" x 30"), I found some air still leaking around the cylinder heads.  I called Gast and the tech person advised me that was basically normal to allow the air to leak off the heads for the next starting sequence.  It was just very fine, very small bubbles around the head gasket. 
The air line to the pressure switch was one of those $*%%^push in type air fittings.  Now changed to DOT compression types (1/4") with NO leaks. Also, a weatherhead socket is handy to remove the brass fitting off the switch.  Tight fit and the screws would not release the bracket. I bought mine at Sunex, on Amazon, of course. I added a air/water separator to the compressor output hose (3/8").  I didn't have the space anywhere else to install a slightly larger compressor.  If anyone has another Gast cpompressor laying around, I'd like to buy one for a spare. This one only lasted about 34 years.

Where was the outflow check valve located??  What does it look like?.  I may have the same problem