Great compressor for years, until it finally wouldn't hold air for very long. So I bought a new electrical ON/Off unit. Still wouldn't hold air. Compression ring was wore out, so I bought compression ring kit SKU  K-0650-SBD with ring, sleeve and gaskets. Still wouldn't build up more than 30 lbs and would keep running. Then I bought their unloader check valve SKU A19711-SBD. Still wouldn't build up more than 30 lbs, no matter how long it kept running. I bought all this from MASTERTOOLREPAIR. Is their products inferior? What should I do? What could be wrong?