Unloader valve does not close?

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The unloader valve does not close. How come? That’s the question John Quincy Adams posed to this website a few years ago. Now, we are impressed with that! As many know, John Quincy Adams was the 6th president of the U.S. and to have him write in to this site from 1848 is pretty remarkable. 🙂

We hope our answer does this august gentleman proud and, even if you aren’t a president, your compressor may be having this issue too. Here’s what to do about it.

First just a quick review of what the unloader is and why, when it doesn’t work, the problems it can cause on an air compressor.

What’s an unloader valve?

Depending on the make and model of air compressor you are having problems with, the unloader mechanism be as simple as an air hole in the line to the tank, or like many air compressors, the unloader valve can be found attached to the side of the pressure switch. This is the one in question and the problem that John was having with his. See the brass tube and fitting on the side facing you in the image below that shows the unloader valve on this Lefoo pressure switch.

lefoo Quality Air Compressor Pressure Switch Control 95-125 PSI 1 Port w/Unloader

When an air compressor stops when the tank is full, it is possible that air can be trapped in the pump. That air can exert pressure and make it very difficult for the motor to start when it has to overcome that air pressure already in the pump. That’s one issue.

Unloader valve does not close?

Another unloader valve problem, and the one John wrote in about, is that the unloader valve is supposed to “unload” the load on the motor. It does this by opening a path from the pump compression chamber to an outside opening to vent the compressor.  Any air trapped in the pump can escape when the unloader valve opens.

A problem occurs when the unloader valve is open during the time the compressor is not running and compressed air also flows out of the tank.

Then, the tank loses air, the compressor re-starts, builds enough pressure to stop again, the air leaks out of the unloader valve from the tank all the while, tank pressure drops and the compressor starts again to rebuild pressure, and so on and so on.

This is not good for the compressor, causes additional expense due to the power wasted to run the compressor for nothing, and even that the user may not have enough air to do work with the air tool.

Why is air leaking out of the unloader valve all of the time?

Why is the air leaking all of the time? To understand that is to understand why the air stays in the tank at all.

Where the air line from the pump enters the air tank there is normally a non-passing valve installed. Air can get into that tank, but cannot flow back out of the tank through that same port as the non-passing valve only allows air in, not out.

However…. you knew one of these was coming, didn’t you. 🙂

If the flap or check ball inside the compressor tank check valve fails to close properly, then as soon as air stops going into the tank, the air in the tank starts to flow back out.

If the unloader valve is leaking…

If, when the compressor is stopped air can be felt coming out of the unloader valve (or out of the bottom of the pressure switch if that switch has an integral unloader inside it) odds are pretty good that the check valve isn’t stopping back flow of the air.

Solution to a leaking unloader valves…

Dump all of the air from the tank with the compressor powered down, remove the tank check valve and try to rinse it to clean it. Maybe boiling water will work, maybe paint thinner or varsol. Rinse it in some type of solvent might allow whatever is blocking the check portion of the valve from working to dislodge.

Let the tank check valve dry thoroughly, reinstall it, and fire up the compressor.

Did it work?

If not, consider replacing the tank check valve since, with the compressor off, the only place air could be leaking from is the tank, and it’s the tank check valve that has the function of keeping that air in the tank so it can flow down the air hose to the air tool when required.

By Ashley Pearce

As a passionate manufacturing and mechanical engineer, I've had my fair share of run ins with air compressors and compressed air systems. With over a decade of experience in the industry, I have both a fresh perspective and time-served hands and mind to help you with your compressor problems (along with our able community!)

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