Can I Plug The Line To The Pressure Switch

This question popped up on a forum recently; ” can I plug the line to the pressure switch “?

The short answer is no, you cannot. Well, actually,  you can if you want, but then your air compressor will not work properly.

You see, there is absolutely nothing installed on a smaller lower cost air compressor unless it’s absolutely necessary. Otherwise, the air line from the tank check valve over to the pressure switch would not be there at all. Plugging that line would, in effect, make it disappear. Not good.

And why would you want to plug the line over to the pressure switch anyway? The questioner said that they had broken the line, a new one was $70, and they did not want to spend that kind of money on a replacement.

Here’s why you should not plug that line, followed by a lower cost solution.

What Line?

What line are we talking about? This first image shows the line in question. It’s connected on this compressor to the base of the pressure switch. It is the copper tube marked with a red triangle.

Air line to pressure switch -

This next image shows a similar line and its source. This line, identified by the red triangle, is aluminum.  It could also be copper as is shown in the first image.


The line plumbs from the fitting where the air line from the compressor pump enters the tank. The larger aluminum line coming in from the left in the second image is the feed from the compressor pump. The fitting into which both are plumbed – the brass “thingy” bottom center –  also usually contains a one-way or a check valve. The air line we are talking about runs from that fitting over to the pressure switch and is indicated by the red triangle.

unloader line from tank check valve - fix-my-compressor.comWhat does the line do?

Because the line is connected to the fitting shown, air from the pump can flow both into the tank through that fitting, and out from the fitting along the smaller air line to the pressure switch.

A pressure switch, to which the line is connected, typically contains an unloader valve either as part of the switch, or mounted on its side. The line from the tank check valve plumbs over to the pressure switch, and it connects to the unloader valve.

When air is being pumped by the compressor pump, it flows down that big line to the fitting, and also flows over to the pressure switch via that smaller line.

Since, typically, while the compressor is running the unloader valve is closed, then the air travelling over to the switch is blocked by the unloader valve and cannot escape. The compressed air, then, flows only into the tank, increasing the pressure in the compressor tank, until that pressure level reaches the cut out pressure setting of the pressure switch. At this point the compressor stops.

The Compressor Unloader Valve Opens

Typically, when the compressor reaches cut out pressure and stops, the unloader valve then opens. Why?

If there is air trapped over the piston in the compressor pump, when the compressor tries to start next time the pressure switch sends power to the pump, that trapped air adds significantly increased load on the piston, and often the compressor motor cannot start against that added load.

The compressor typically goes off on thermal overload, or trips the breaker / blows the fuse.


If you were to plug the line over to the unloader valve, the next time your compressor tried to start at cut in, quite likely it would not. The trapped air over the piston would prevent the pump from cycling properly.


If the cost of an OEM line from the check valve to the pressure switch is prohibitive, don’t replace it with OEM equipment.

What you can do is visit your local compressed air fittings supplier (Google industrial air cylinders – these folks have fittings) and acquire the fittings necessary to replace the hard metal tube with a polyurethane tube.

Instant fittings, to which the polyurethane “quick connects” make the process fairly simple.

You will still need to spend $20-$30, but that’s better than $60-$70 for an OEM part.

The only thing you need to be aware of is that, if you are using your compressor for long periods of time (as in you use it to make money), then replace the part with the OEM metal tube. Polyurethane may not stand up to the high heat that is typically generated on a compressor when it is being run for extended periods of time.




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