Pressure won't build up in tank

Your compressor is the oilless type that incorporates a vinyl/teflon piston ring and a cylinder that is open from the bottom. Remove the cover and push the piston to upmost position. Look for scratches along the upper portion of cylinder. If scratched, compression is leaking thru ring and will not fill tank. Replace with Sears part #k0650 cylinder /piston ring kit to restore full power. While the compressor is apart examing the valve plate and reed valves for wear. If worn replace with part #z-ac-0032. Reed valves tend to crack or loose their shape reducing efficiency. If cylinder and ring are in fair condition, then most likely reed/valve plate is the problem. Good luck and email if you have more questions.

If your air compressor builds pressure to a point, and then won't go any higher, check your gaskets. If you take the compressor apart, be prepared to replace the gaskets as they are brittle after use and most likely will tear.

Check the gasket that divides the chambers by the air intake. Make sure that there is no part of the divider gasket missing. Even a small gap can cause pressure to cease building. You can buy replacement gaskets for most brands, or you can buy a sheet of gasket material at an auto parts store for under $10 and cut your own. Try to keep the old gaskets in tact as much as possible to use them as a template.