Quincy Air Compressor Help needed

A few months ago, I bought a used Quincy 3-1/2HP 220 air compressor for my new shop. About 5 years old, but only had about 20 hours on it. It ran fine at the seller's house. About a month ago the electricians finally wired my 220 outlet for the compressor. It ran fine then, too. I attached some piping for an outlet, tested it, it pumped up fine. A few days later, I noticed I'd left the switch in the auto position, the pressure gage said zero, and it wasn't running (although the hour meter was - it now said 83 hours). A little internet research pointed me to the pressure switch, plus it was relatively cheap, so I bought one and replaced it. Same result. At that point I checked the old switch, and it seems to have continuity when it should, both sides (black and yellowish white wire). In retrospect, the fact the hour meter was continuing to run should have told me it wasn't the switch.

That pointed me to the motor itself. Further disassembly, the motor turned by hand, as did the pump, although I didn't go very far with the pump - it quickly got hard to turn by hand, and I figured I was on a pressure stroke, so I stopped. Took the motor to a motor shop, a wire had come off internally. They fixed that, checked everything else, declared it healthy. Tonight I put it back together, turned the switch, and quickly heard a screech. Figured it was belt alignment or the guard rubbing. Shut it off, took off the belt guard, tried again. Still screeching, but noticed the pump isn't turning. Took off the belt, I can't turn the pump either direction by hand. it had built up a little pressure, but not enough to register on the gauge.

Am I just unlucky, or did I do something wrong? Anybody got any advice?