Quincy Air Compressor Help needed

Agree with post #5. There is a check valve that prevents the crankcase from pressurizing usually right where the air goes in the tank. If relief valve is bad it doesn't release the pressure the motor can't overcome the pressure. Like said most have an unloader valve at the pressure switch but not all. Some have them on the compressor head. Any pics of the Unit?
Check your check valve and bleed down said in post 5.