Air compressor tripping 20 amp breaker

Model number 919.152920 I have a Sears 25 gallon/4HP horizontal compressor. A week or 2 ago I posted that when this compressor reached 125 PSI it shut off like it should, but when the pressure dropped and the compressor kicked on again it would pop the breaker. The suggestions made in my previous post said the unloader valve is bad, or check valve is bad. I looked in the parts list and I do not see an unloader valve listed in the parts list. I do see a restrictor check valve. I just took the 2 covers off of the unit to take a look at everything. I thought I would kick the compressor on to get it up to pressure and now I can't even do that. It immediately pops the breaker when I turn it on. I started experimenting and the first thing I did was to remove that the small tube/pipe that connects to the switch and to the tank plumbing. I then turned the compressor on and it runs fine, so at least I know the motor is good. What do you folks think my problem is now? Do you think the switch is bad that manually turns it on or off? The parts list calls it a pressure switch. I want to thank you all again for your help.
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The unloader valve relieves the pressure in the compressor after it reaches trip pressure.
A properly functioning unloader will have a noticable hiss after the compressor shuts off. No hiss - bad/clogged unloader.
Dave

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When the compressor is fully charged and turns off you should hear a brief hiss noise. This is the pressure relief valve letting the pressure off the pump. If you do not hear the hiss sound, the pressure relief valve is not working properly. The reason that you are popping the breaker is because the pressure has not been released and the compressor is trying to start up under a load. This valve is usually located where the small hose going from the switch and the larger fill hose meet. Typically it only looks like a brass "T" fitting.

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Are you running this thing on 115 or 220? I have a similar compressor that would trip the breakers on startup at 115. It is a big motor and the draw current to start is pretty high. I switched it to 220 and the problem went away. max

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should,
If I recall your compressor would run fine starting from a completely discharged tank, it would trip the breaker on restart when the pressure drops and the pressure switch turns on the motor again.
You have an unloader some where, it is bad! Greg
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should,
unloader
when
tank
Look in your manual for the check valve. It is at the end of the tube going into the tank. This is the unloader. Take it out and clean it in a solvent or gasoline. The valve disk should move freely and make sure the spring holds the disk closed. Put it back together. If it still does not work - replace the valve. These valves often get carboned up from heavy use and most times just need a good cleaning.
--

-Mike-
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should,
unloader
I just looked at the parts diagram on line. The pressure switch has an unloader built in to it. The pressure switch screws on at it's base, this is where it senses the pressure in the tank. There is a small tube that attached to the side of the pressure switch, this tube is to unload the pressure from the compressor pump when it shuts off. My best guess is the pressure switch is shot, the unloader potion of the switch has failed. Greg
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Thanks Greg for your response. I think you are right. I took the switch a part and there is no way I can repair the switch. It appears that the little lever on the bottom of the where the tube screws on does not close all the way and looks like it doesn't move at all when the manual switch is tripped. Inside the switch where the 4 wires hook up there 4 points. maybe they are pitted too. I'm going to go ahead and order a new switch and hope that cures it. Thanks again everyone.

would
do
is
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little
tripped.
are
Try cleaning it first Ace. The unloaders will develop a buildup from the compressor oil and stick. Most times a fast cleaning with a solvent fixes the problem. This is a very common occurrence.
--

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