compressor malfunctioning

[Mr.Hyde]

I'm having a bit of an issue i purchased a compressor used locally and it seemed like a great deal but now its starting to act up. When i go to start it from 0 pressure, it builds up pressure to about 125 psi but if there is any sort of pressure in the tank the thing just hums and the pops the built in breaker. what could be causing this its a central pneumatic 21 gallon model #47065. any advice would be appreciated.

[truckdude239]

its cheap and wore out? they sell those at harbor fireght all day for 150 buxs

[vidman]

motor is not strong enough to start under load. what breaker is it tripping, panel breaker or one on motor. might be time for an upgrade

[Mr.Hyde]

One on the motor I did some research a guy replaced his unloader valve then worked great.
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[ETsC10]

Keep us posted on this.
Nice to see it might have a fix.

[ERASER5]

Yep, there is an "unloader" valve. That is the short hiss you hear when the compressor reaches cutoff. The unloader drops the pressure from the head to the tank's check valve. If your check valve is malfunctioning, the compressor will continue to his until restart or empty if you turn the power off.

The pressure controller spring cuts the power contacts and a little tang pushes an extra long shrader valve dropping the head pressure to zero so at startup, there is no load. This is a standard problem with used compressors being "rigged" to not leak. Like mine was.

Your check valve is inside the can. The pressure pipe from the compressor head connets to a brass (most likely) nut at the tank. that is not a bung or flange but a check valve. It unscrews from a bung and is about 1-1/2" to 3" for big units.

[Mr.Hyde]

i took some pictures not too sure if this will help.

[dwcsr]

I would try the unloader first. It sounds like the compressor is under pressure and the motor isn't strong enough to spin it. You should hear some air release when the compressor stops, thats the unloader working. If you don't hear it change it.