I have a four year old Sears 4 HP 25 gallon air compressor (Model No. 919.152920) that will no longer pump up to it's full capacity of 125 psi. It pumps up to about 80 psi and just stays there never shutting off. I've replaced the regulator, top manifold, check valve, flapper valve, and valve plate assembly. The problem remains the same. Any idea what might be the problem? I suspect it might be the motor or piston assembly but I don't know how to determine for sure and a new motor would be too expensive to replace. I have ruled out the pressure switch as the problem because it is not supposed to shut off the compressor until it reaches the factory cut-off pressure which is much higher than 80 psi.