All the air comes out of the coupler

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“When I fill the compressor with air. I take the hose off the coupler, all the air comes out of the coupler.  What is wrong? How do I fix this? The Compressor is a Central Pneumatic, Oil Air Compressor,  8 gallon 2 HP 125 PSI. Please…”

…so writes Carol recently and we are happy to reply as follows.

We are talking here about what is commonly referred to as the discharge coupler, a device on the air pipe from the compressor into which the connector on the air line is plugged. When a connector is plugged into the coupler, air is able to flow from the compressor tank, through the air regulator, out the discharge coupler and into the air line to the air tool.

Compressed air coupler showing where the connector is inserted.
Compressed air coupler showing where the connector is inserted.

When the air line connector is removed from the coupler, whether there is little or a lot of air in the compressor tank, a small “flapper” or “ball check” inside the coupler should be immediately forced closed both by the air pressure remaining in the compressor tank or shut tight by the integral spring inside the coupler. The spring and ball are forced open when a connector is inserted securely into the coupler.

Depending on the amount of water and air borne crud coming out of the tank (quantities of which are not good for the regulator either) the coupler “closing mechanism” may become coated with, or “glued” open by this debris.

In particular, if the compressor has not been used for a long time, this “stuff” dries inside the coupler. When the connector is inserted the force may open the ball check, but the amount of debris may actually cause it to stick open, and that is why all the air comes out of the coupler when the connector is removed.

It’s that or the coupler has, in some other manner, shape or form, failed with the internal spring rusted out or broken. Otherwise, it’s the coupler is likely just “crudded” open.

How is this resolved?

You could spray a bit of lubricant up inside the coupler first in the hope this will dissolve some of the crud and allow the ball or flap to shut when the connector is removed, if it’s that and not a broken spring that’s allowing the check ball to stay open.

Or, shut down the compressor, dump all the tank air, and remove the coupler. Drop it in a pot of just-boiled water and swish it around a bunch to try and get hot water inside the coupler to clean out the debris.

Let the coupler dry thoroughly, and hit it with a little lubricant. Just a very little though, as lubricant blowing into the air line to the tool may not be a great thing.

Reinstall the coupler, start the compressor, and see if that’s allowed the coupler to shut and keep the air inside as the tank pressure grows.

If it did, problem solved. This being the case, with a full tank of air, open the tank drain and allow all the water and debris possible to blow out.

If it did not work, you might try the whole scenario again or accept that maybe the ball check or flap spring inside the coupler has failed, and replace the coupler with a new one.

If the latter, make sure the coupler is of the same make as the connectors being used.

Good luck, and please let us know as a comment if this helped.

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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