1991 BlueBird all American- air brakes

[magnakansas]

I am still absorbing information about air brake systems.

The problem - low maximum air pressure. System seems to run up to 75 psi and holds there.

I suspect the air compressor governor is the problem. Either stuck or adjusted low.

What are you thoughts on this matter.

William

[o1marc]

It should go to 120psi before purging.

[rossvtaylor]

Could you have a blown airbag or something else which is draining air from your auxiliary air tank/system? A major leak in that aux system would cause exactly what you're experiencing. The aux system is separated by a pressure protection valve and, if there's a big leak in that system, the main brake tanks won't go get above the PPV setting...which is often in the 60-80 psi range. So, if your PPV is a 75 psi valve then your brake tanks will get to 75 and then stay there, while the excess air pressure is leaked out through the aux system. We had this happen on a transit bus... It was a blown airbag.

[magnakansas]

Suppose that is possible. This bus does have four bags for the rear, they are all up. I do not hear leaks at all. Until I find the cause, or causes, everything is possible. That gives me some things to ponder.

William

[Booyah45828]

When you hit 75, does the drier purge and exhaust out the bottom port? It's the PSHHHT sound you would hear from the drier when the vehicle would normally reach around 120.

If it doesn't, your governor is fine, and your problem likely lies with something in the auxiliary system like rossvtaylor alluded to.

[magnakansas]

I have not heard a purge yet. Going to try to isolate rear air bags and evaluate the system.

William

[rossvtaylor]

Originally Posted by magnakansas

I have not heard a purge yet. Going to try to isolate rear air bags and evaluate the system.

William

Bags tend to wear most where they are hardest to see, in the fold. If you can fill a spray bottle with water and some dish soap, then spray all bags and hose connections and fittings, you should find any leaks. The bubbles should be obvious!

[magnakansas]

I have been working on a 1991 blue bird all american front engine. Air pressure would only hit about 75 psi. I did not hear any leaks. I finally go under the bus and went over the system again. We even adjusted the governor settings. Improved, but still low. So low never got the purge valve to open. Funny thing is the purge valve area was very clean like it had worked until very recently.

Pressures were sitting at 75 psi. From 0psi to 75 psi happens quickly which led me to believe that the compressor was in good health and would be able to get the job done. I just had to find why we were limited to 75 psi.

I had my head up under the bus right under where the driver sits yesterday.

I found the cause of the low pressure.

The air line to the air ride drivers seat was open and venting air. We reconnected the air seat and the problem was almost solved.
We adjusted the governor to bring maximum pressure from about 105-110 up to 120.

Now I am happy with the air brake system.

I could not hear that open line venting pressure very well at all. I had to have quiet all around me to hear it. Engine start, run up pressure to maximum - 75psi then engine off and quiet. It was really faint to hear, but the hose was only about 24 inches away from my head, and right in front of my face. When I did hear it, I had to swivel my head around trying to locate the direction of the leak so I knew kind of where to look.

Drain valves on the all three air tanks are working very smoothly and one tank had quite a bit of water in it. Very tight system right now. Air bags hold air after sitting two weeks.

Thanks for all your input.

william