Compressor blows breakers when warmed up with air in tank
First time posting, appreciate any help.
I have a DeVilbiss PRFB5520VP-WK 20 gallon 5.5 hp oil-free compressor. I bought it on Craigslist, and I can see why the guy sold me this lemon: it's having motor start issues. Basically it trips 20 amp breakers when attempting to start after it's warmed up. It doesn't trip a breaker every time, but 1 out of 3 times or so that the motor starts.
For diagnostics, I resorted to YouTube videos.
I started by rotating the motor by hand, and there's no binding or noticeable rub or wear in bearings. I cleaned the start circuit contact breaker with sandpaper, there was a bit of burnt corrosion on it, but nothing terrible. I cleaned the 4 pressure switch contacts with sandpaper, as well. I bought a new piston, connector rod, cylinder, valves, and seals kit and replaced them all. The previous cylinder/piston had some bad wear that made the piston seem to bind, so I really thought that would fix it. Still blowing breakers. I bought a new start capacitor for it with about 10% microfarad rating increase, hoping that the start capacitor was bad. Still blows breakers.
What's left to diagnose? At this point, my very limited electric motor understanding is pointing me to the motor windings going bad. Any ideas? Many thanks in advance.