I have a husky air compressor model WL651004AJ will only build up to 50 psi and compressor keeps running any advise som one could give me to whats wrong.

You can inspect the check valve by removing it and pulling on the small plastic pin sticking out of the middle of the tank side. It should spring back under fairly light tension. Also visually inspect and clean out with a dry q-tip the brass screen on the inlet site. Seal back with ribbon dope.

If this looks ok, remove the 4 bolts holding the head on. Keep all of the sandwiched plates together in order so they go back in easliy. The cylinder sleeve will come off with this stack. Look at the plastic surround on the piston and the inside of the cylinder. In my case, this was eaten up from too many hot summers and cold winters in the garage. The rubber/plastic had hardened and eaten part of itself away. To replace the entire piston assembly on the WL651004AJ, you'll need to get part number WL211000AJ, which in some books is superseded by WL211000SJ.
If the cylinder is scored (meaning that by scraping your fingernail against the lines in it, you catch the nail) then you're probably better off replacing the whole cylinder unit.

Two possible common things:

1. Check valve is bad or has contamination. It is often located right where the pipe from the compressor goes into the tank.

2. If the thing has an unloader valve, the valve fails to close. The valve is often where the pressure switch is and if this is the problem you can hear it hissing from that area.