This one is pretty easy, you have either defective reed valves in the pump, or a worn piston (compression) ring and cylinder wall.

In oil-free units this is typical, the teflon-coated piston ring probably has a cut, tear or rip along the perimeter which is causing air to pass by so it's not getting a good seal in between it and the cylinder wall.

There is a Piston/Rod Kit available that includes all of these parts for around $41 (the cylinder is sold separately at $11), you can view it by clicking on the below link:

www.mastertoolrepair.com/air-compressor-...-pc1010-p-75008.html

I would take the pump head off and inspect it first, of course, but most likely you'll find the Piston Ring/Cylinder and/or reed valves to be the reason why it won't fill up. If defective, the pump will conceivably run all day because it can't reach the maximum pressure that the switch is programmed to shut the unit off at.

- MTR