1 hp compressor motor

My 30 yr old portable compressor (1 hp, 12 gallon) starts and runs when "empty" but when the pressure switch kicks on at 80 psi to rebuild pressure it just buzzes and trips the breaker in the shop. This is a Doerr motor. Suggestions on what to look at? I didn't realize how handy it is until I don't have it...
Thanks in advance. Tim
 
First thing I would look at is the check valve to make sure it is going closed. Then I would listen for air bleeding off when the pressure switch cuts out as it should bleed off the air in line between the check valve and the compressor allowing it to start under no load.
 
The capaciter is shot. Got one here just like that. Starts just fine and as soon as it starts to build pressure it blows the fuse.
 
(quoted from post at 19:56:25 02/05/16) The capaciter is shot. Got one here just like that. Starts just fine and as soon as it starts to build pressure it blows the fuse.
Doesn't sound like his problem. His will pump up to pressure, but then not restart, which sounds like an unloader problem, as others have said.
 
Thank you all for the comments. I'll have to go out and listen for the hiss of the unloader valve. I'll report the findings and thanks again for all the help!
 
After it shuts off you could loosen the line between the compressor head and the check valve, if it hisses and stops, it's probably the unloader, if it leaks continuously it's probably the check valve.
 
The unloader valve isn't working that will cause that problem. The unloader valve located where the line goes into the tank should bleed off the pressure between the tank and the compressor so it can restart with no load on it but yours is probably starting under a load is why it doesn't want to turn.
 

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