GM Air Compressor Exhaust Muffler Check Valve

[richard5933]

When shutting down for the day, I drain my tanks. I also drain the petcock on the compressor exhaust muffler. There is usually a brief blow of air and water, then it stops. If I open the petcock again, there is nothing. I always assumed that the check valve was doing its job.

Today I noticed that if the bus sits for a few days and then I turn the petcock again, there is a small poof of air (much smaller than initially) and a little more moisture.

Am I correct in assuming that there is a little air getting past the check valve? Is this little amount of air a problem that needs attention? (It's noticeable on the air gauge when doing the pre-trip air brake check.)

I checked the manual and parts book, and they show a check valve just after the exhaust muffler. All I see is an elbow, and then a hard metal airline curving to a fitting on the engine bay bulkhead.

Is the check valve in this elbow? If not, where is it located? On the other side of the bulkhead on the rear axle side?

[chessie4905]

My 4905 has a large check valve right at the main air tank in the line above the batteries. About 1" dia. They list a repair kit, probably no longer avail. I replaced mine when I went over the complete air system last year. Everything is rebuit or replaced. The tank btw is split with a check valve between them. You remove a cap on side of tank to service it.

[richard5933]

My 4905 has a large check valve right at the main air tank in the line above the batteries. About 1" dia. They list a repair kit, probably no longer avail. I replaced mine when I went over the complete air system last year. Everything is rebuit or replaced. The tank btw is split with a check valve between them. You remove a cap on side of tank to service it.

Is that the same air tank I have in the HVAC cabinet? Just behind the passenger door? Mine is also a split wet/dry tank.

The diagram in the manual make it sound like the check valve is right next to the muffler. What's strange though is in the parts manual sect. 4-55 there is a note to see sect. 4-27 for the check valve. When I look in 4-27 the only check valves are for the tanks. Maybe that's the one they're talking about.

My main question is still open. Is that small amount of air seeping through the check valve over two day enough to even worry about right now?

[dtcerrato]

Richard we have different busses but they're both GMs - so somewhat similar. There is at least 1 check valve between your air compressor discharge muffler and the first (wet) tank. The fact the check valve lets a little air through is normal. On mine the check valve is a metal disc that rests against a metal seat. The primary function of the valve is to "save" the volume of air in the system in the case of a catastrophic failure of the compressor discharge line which takes the brunt of tremendous heat. Opening the muffler drain & hearing a little hiss then nothing is normal. Mine has done that forever, I wouldn't let it worry you...

[buswarrior]

X2. The check valve is there to protect the system for operational use, not storage use.

In principle, the check valve goes against what it is protecting, the tank, otherwise the line leading to the tank becomes another weak link.

Keep looking for better things to fix, leave this one for when there are none left.

The moisture will be what was on the walls, it finally ran down to the bottom over time.

Bathtub isn't dry when you drain it...

Happy coaching!
Buswarrior

[richard5933]

Thanks for the confirmation guys. I didn't think it was anything to stress over, but I thought I'd check so I didn't miss something while it was still a small problem.

[chessie4905]

Here are pictures of mine. The hex plug at top of pictures is the between tank half checkvalve. Has a flat disc, spring and seat. The big line lower left next to brass fitting is main checkvalve (silver grey hex). That line goes to back of coach to air compressor. There are water drain valves for both halves of tank inside the fuel filler door. 90 degree turn of handle. My parts coach, which is a P8M-4905a has push valves similar to parking brake valve, but just a spring loaded air release valve when pushed. There is also a low pressure warning switch just to right of pictures. It was leaking. Easy to rebuild these. Luke had kit for them.

[richard5933]

Here are pictures of mine...The big line lower left next to brass fitting is main checkvalve (silver grey hex). That line goes to back of coach to air compressor. ...

That main check valve between the back of the bus and the tank - is that something that you sourced locally or was it a specialty item you got from someone like Luke? Seems like it ought to be standard, but you never know.

Just asking for the inevitable day mine does need replacing.

[chessie4905]

Bought it on Ebay. Matched fitting thread size.