Porter Cable Compressor rebuild

[dan1]

I've a Porter Cable C3151 Job Boss, oil-less compressor.
I replaced the valve plate, piston ring, cylinder, pressure switch and start capacitor.
I had the run capacitor and start capacitor tested and they're ok, but I replaced the start capacitor anyway.
When there is no pressure in the tank, it starts right up, runs to cycle and shuts off perfectly. When I drop the pressure to begin a new cycle, sometimes it will start right up again no problem, and run the cycle. But, sometimes it will labor at start and throw a 25 amp breaker. The only way it will start like this is to let most of the pressure off and turn the switch back on (after resetting the breaker).
Sometimes I've been able to cycle it a dozen times in a row, then for no apparent reason (to me) it labors and throws the breaker.
I've thought about this for a long time and wonder if there is a problem with the motor.
When a car's starter developes a "flat spot" it labors to start the car. Pushing it in gear will move the starter and it will turn the motor over.
Is it possible that this motor has developed a "flat spot"? Could this be why it labors some of the time when starting and not other times? Does this motor have brushes that could need replacement?
Any ideas would be greatly appreciated. I can't justify spending the price of a new motor, I could buy a new compressor for far less...

[speedyairrepair]

Most oilless compressor or most porter cable do have brushes. But it sounds like your check valve is leaking. next time turn it on and leave for a minute then try to turn it back on. If it struggles or pops breaker that's the problem.

Any more ? just post. I've fixed over 100 of the same model and type.

[dan1]

thanks for the reply
I proved that it was the check valve in the following manner:
1. I let the compressor fill to maximum pressure.
2. I unplugged it.
3. Then manually I tripped the pressure switch to release the pressure in the head.
4. When I noticed that the pressure continued to bleed for minutes at a time, and that the pressure in the tank was dropping, I knew the check valve wasn't doing it's job.

I await a new valve, confident it will solve the problem.