[Jrahn56]

I have coach #375, after running at high idle for 30 minutes I have only 70 psi built up in brakes and supply. When I shut the engine I immediately lost my supply pressure with 3 minutes. I have drained all the tanks I could find, but that was 1 for the 12v compressor,  1 in the cross member by drive axle, and the brake tank that had a considerable amount of water built up. My coach utilizes a CR Brakemaster Model 62 air dryer. The air coming into the dryer is hot, the going air is cooler, but pressure coming in doesn't seem enough. I can hold my thumb over the fitting and stop the air. I've tried to get the air governor off the front of the compressor but the bolts are frozen tight. I've tried penetrating oil, but no luck. I wanted to try heat, but didn't want to damage the compressor. Can anyone help diagnose this problem?

Thanks,

Jeff Rahn

[cwl1979]

Best guess would be the pressure protection valve went out on you. Common part and cheap.

[Richard]

Ok, the loss of supply pressure in 3 minutes indicates you have a huge air leak. That must be found and repaired before you make any other conclusions about your problems. One step at a time.

You may not be able to hear the air leak with the motor running. It can be very helpful to run an outside air source to the port in the engine bay to provide air while leak hunting.

The air compressor governor MAY be a problem, but before figuring that out, fix the huge hole in the system.

[360]

I had something similar happen this year and it was a hole in the air line that runs from the compressor to the dryer. As Richard said you can't hear it with engine running. If you can hold your thumb over it that would be my guess. In my case there was ZERO shielding on that hose as it draped over the bell housing area. So that how was allowed to just sit there and rub for 20 years until it had a nice hole. Newell does so many things extremely well, but in general shielding on pretty much everything from my era was all but non-existent so you may want to take a good look every line when you get to poking around and have a little shielding party.

[Richard]

Jeff,

You sent me a PM asking some questions. I will answer them here so that others can also help and learn.

I am assuming this is a tag axle coach. In the rear you will find 3 air tanks that are square and welded as part of the frame. If you follow the air lines in and out of those tanks you will see that they feed the air bags for the drive and tag axles. The rearmost tank is actually divided into two tanks internally to feed the right and left tags. They normally have square plug fittings in them. You will also find one round tank. That is the wet tank, the first tank in the system after the air dryer. It should have a drain of some sort on the bottom.

In the front, you will find square tanks welded as part of the frame. They are connected to the right and left air bags. You will also find two round tanks, they are the primary and secondary brake tanks.

In addition you will find one small tank in the right side drive wheel well in front of the drive tire. That is a supply side tank, make sure you drain that one. You may or may not have an additional tank connected to your 120V air compressor.

I really suggest you plug whatever you have disconnected and find the big leak first, the big leak causes all kind of confusion is diagnosing other problems.

I find it useful when a compressor is suspect to deadhead the compressor with gauge. Be aware that if you do that on the engine compressor, you should be prepared to hit the kill switch in the engine bay when the compressor climbs to 120.

Air system problems can be very tricky to understand when there are multiple issues. I find that thinking about how to isolate different parts of the system and pressure test is an effective approach.

These videos may help you. https://newellgurus.com/showthread.php?t...9#pid42329

[CaptainGizmo]

Jeff,

As an adjunct to Richard’s suggestion…

Believe it or not, the AMPROBE leak detector CAN find leaks when the engine is running. I was able to successfully find a leak on the engine compressors’s output fitting.

Expensive, but I dispensable tool of you’re interested.

https://amzn.to/49uQSMf

I’ve made a few videos about using it on my YouTube channel “RV Squared”.