Thanks for the post!

I cant' blame you for being frustrated, this symptom can be caused by many potential problems so it's irritating to say the least when it still occurs when you've dumped a decent amount of money into it.

Initially I would have said the same thing, your Check Valve is sticking open after the unit shuts off, thereby putting back pressure on the Head and causing the breaker to trip, but that's obviously not it.

Sure, the Unloader Valve on the Pressure Switch can cause this to happen as well, but you've replaced this also. Just to confirm, when the unit reaches 125 PSI and shuts off, does the Unloader Valve actually blow off and unload the air like it's supposed to? Let's make sure that is actually happening, firstly.

The only other reason this could be happening is due to an electrical issue, but I doubt it.

Make sure there is no debris in the Discharge Tube, which connects the Pump to the Tank inlet. You may have had some debris sucked through the intake that's keeping the check valve open, which can cause this.

I'll try to get some additional info and reply.

- MTR

No problem

You can inspect the Unloader Line you're referring to, sure, but I was referring to the larger (3/8" or 1/2" OD) Transfer Tube that connects from the outlet of the pump head to your tank (Check Valve); this carries the bulk of the air to the receiver tank. It should also have compression fittings on either end.

It's possible some debris was sucked into the intake filter of the head and is causing your Check Valve to stick.

It is also possible that this is an electrical issue with the motor.

If this motor has both a starting and a running capacitor then it is possible that there is a problem with the starting capacitor (the cylindrical object located under the motor shroud, usually bolted onto the top of the motor) or the connections to it.

There is a centrifugal switch located inside the motor that opens and closes to transfer connection to and from the start and run capacitors. If this switch is sticking or not making a connection when the motor stops then the start capacitor will not be connected in the circuit for start up of the motor. I would make certain that there is not a back pressure issue by running the unit up to pressure and removing the discharge line from the compressor pump head. If air leaks out of the Discharge Line from the Check Valve after it shuts off, then you the Check Valve is sticking.

If the unit still will not start then I would proceed to checking the motor connections, capacitors and switch.

Hope this helps!

- MTR

That's correct. Since the Transfer/Discharge Tube is connected to the Check Valve, which is installed into the tank inlet, you should not get ANY air back-feeding from the Check Valve and through the Tube.

The Check Valve should stay closed while the unit is off. If you get any air leaking back, the Check Valve is the problem. If no air back-feeds through the Tube, the problem lies elsewhere.

- MTR