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I can’t locate the check valve on my LeRoi Dresser air compressor

Published Categorized as Air Compressor Troubleshooting, Air Compressor Valves, LeROI / Dresser No Comments on I can’t locate the check valve on my LeRoi Dresser air compressor

by Tim
(Duncannon PA USA)

Hi, I have an older LeRoi 80 gallon air compressor with a Dresser 500 2 stage compressor. It is leaking air through the bottom port of the unloader valve when the pump shuts off. The unloader doesn’t seem to be unloading either. It is locate on the front of the compressor pump.

Leroi Dresser Air Compressor
Leroi Dresser Air Compressor
Leroi Dresser Air Compressor Pump
Leroi Dresser Air Compressor Pump
Leroi Dresser Air Compressor Pressure Switch
Leroi Dresser Air Compressor Pressure Switch
Leroi Dresser Air Compressor Plumbing
Leroi Dresser Air Compressor Plumbing

Reading through the posts on your site it would seem that the best place to start is by examining the check valve. The problem is that I can’t find anything that looks like a check valve between the pump and the tank or between the pressure switch and tank.

There is a copper line between the high pressure cylinder and the tank. On either end of the line is what appears to be a 90 degree gas line compression fitting.

The pressure switch is plumbed into a standard looking pipe “T” which is plumbed to a pipe nipple going to the tank. There is a plastic line plumbed into the pipe “T” below the pressure switch that runs to the unloader valve.

There are 2 small copper lines that run from each of the cylinders to a “T” and then to the unloader valve. Nothing looks like any pics I have seen of a check valve.

Any thoughts would be appreciated. I can send pics as well. Thanks, Tim.


LeRoi check valve
by: Doug in s.d.ca

I’m fairly sure the function is built into the pump, so there won’t be a separate check valve.


Servicing the check valve
by: Tim

So how would I service the check valve? Is it in the high pressure head assembly?

I don’t think the unloader valve is working properly because I tried to turn the compressor by hand after the pressure switch shut it of and it was very difficult to turn. 15 minutes later is was much easier to turn.


Unloader check valve
by: Doug in s.d.ca

I don’t know. However, I expect that once you get into the unloader, the check will become evident.

I’m hoping someone has a good drawing of the pump, as I’m not finding a one.


I can’t locate the check valve on my LeRoi Dresser
by: Anonymous

Your compressor does not have or need a tank mounted check valve.

The second stage discharge valve/s serves as the check valve.

It sounds like the second stage discharge valve/s is leaking. I would inspect it, it may just need a cleaning but might need to be replaced.


Already sold it
by: Tim in Duncannon

Good to know! Thanks. I sold the compressor a while back because I didn’t want to spend $300 on the special tools to take the valves out.

I made my own but it broke when i put a 4 ft cheater bar on it and did push-ups (i weigh 225).

The valve didn’t budge.

I bought a Kellogg 325 mostly because it can be completely taken apart with common hand tools.

I rebuilt it and it works for my needs.

If I had run the Leroi Dresser every day it wouldn’t have been a problem but I only ran it when I ran my sandblaster every couple months.


No Check Valve
by: LarryK

It was normal for Leroi to manufacture their reciprocating air compressors without a discharge tank check valve.

They relied on the pump HP discharge valve to function as a check valve. If the discharge valve leaked it would result in air blowing from the unloader on shutdown therefore indicating a need for valve maintenance.

Leroi felt that having a check valve in the discharge line would mask/hide a valve problem. You need to repair the HP discharge valve and probably check the other three valves.


Compressor Gone
by: Tim in Duncannon

Larry, thanks for the input. I sold the compressor a while ago but it is useful info for others that may be experiencing the same problem.

When I found out I would have to drop close to $400 on the special tools to remove the valves I opted to sell it and buy something I could actually work on. I rebuilt a Kellogg 335 with basic hand tools and a cylinder hone.


Le Roi didnt use check valves.
by: KJ

Leroi didnt use a check valve. The HP exhaust valve served as the ck valve. If air leaks out the unloader, either the bellofram is torn beneath the unloader cover or the HP exhaust valve needs replaced.


Unloader Leak
by: Tim in Duncannon

Thanks for the info KJ. I rebuilt the unloader and installed a new diaphragm so it must have been the HP valve. Your info confirms my decision to sell it. I would have replaced the valve and kept it but, no matter how hard I tried, I could NOT get the valve assembly out. I wasn’t going to spend $150 for the necessary socket.
In other news the Kellogg 335 is running great!

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By Bill Wade

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