Hi, if adjusting the regulator does nothing and it had no leaks the next thing is to pull the head and inspect the pressure plate and reed systemPressure plate rebuild parts are about $25
remove the 4 bolts holding the head on top of the cylinder. Remove the head.
Directly under the head is the pressure plate. The pressure plate reeds keep the air from going back through the intake side of the compressor. Check the reeds for warping or breaking. If the reeds are damaged, you must replace the pressure plate with it.Make good notes of how the pressure plate sets and the o-rings that seal around it.
Remove the pressure plate and inspect the cylinder walls for scratches or wear, if the cylinder is damaged you need to replace the cylinder, piston, and rings. Piston/ring/rod rebuild kit $35If a ring is worn or broken the air goes back through the intake filter and no pressure is built. If you have to replace the rings, I would replace the piston and sleeve too.
If you do not feel confident repairing this problem yourself that is fine but I wanted to offer up at least how and what to inspect.
here is the breakdown of partsClick hereParts can be bought from sears parts direct website.