compressor wont start

I have a 6 year old campbell hausfeld 28 gallon 6.25 horse direct drive oilless air compressor that has seen minimal use. I had it in storage for 3 years and used it for the first time the other day.

I started it up and let it build pressure to about 100 psi then shut it off.

I later turned it on again, but now it only just starts to turn the motor, then hums and trips my breaker.

The piston moves freely by hand. Could it be the starter capacitor? There is currently no air in the tank and still wont run. Thanks for your help.
Steve

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Bill says...

If the motor starts the compressor, then the motor is starting, so I don't think it's the starter.

"I started it up and let it build pressure to about 100 psi then shut it off. " How come? Did it get to cut out pressure then? Just curious as to why you didn't let it completely cycle?

Bill