Sounds like you're already well familiar with all this but just for the record in case someone isn't:::: The "check valve" allows air flow FROM the compressor TO the receiver tank but NOT out of the tank back to the pump (Its NOT a head pressure unloader),,,,,,The "pressure relief valve" pops off if pressure is too high,,,,,,The "Unloader valve" is supposed to bleed off pump head pressure allowing for an easy unloaded start up but NOT of course, bleed off tank pressure.
So I would suspect a bad unloader buttttttttttt you stated "I figured it wasn"t releasing head pressure, but that checked out fine."""
If its NOT a pump or unloader problem next Id suspect a bad start capacitor (ifffffffff its a single phase split phase capacitor start motor but 10 HP is awful big????) if the motor is a capacitor start motor but sounds like you replaced a capacitor, was that the start (if single phase cap start) or run capacitor (If your motor is so equipped)
If its NOT an unloader problem or a capacitor problem, ya know if normal (can be 5 x running current) surge start up trips a regular circuit breaker sized to protect the feeders, the Code as I best recall allows the use of a higher amp rated breaker which allows a motor to start!!!!! The motors thermal overload protection device protects the motor while a short will still trip the circuit breaker and protect the feeders. Or if its a fused unit you can use dual element time delay (Class K5 if I recall) fuses, they will allow start up,,,,
Im rusty as an old nail having been too long retired as an EE but my friend the Ol Dusty man there as well as several other fine electricians techs and other engineers and sparkies here might add to or correct this
Ol John T and all