[Glenn]

The relay valves in the brake system get a signal from the foot treadle or parking brake valve and open the path for air flow from the air tank to the brake chambers. When that air signal is gone, such as taking your foot from the brake, they function as a quick release valve and exhaust the air that applied the brakes.

The ABS valves are getting a signal from the ABS system sensors and control and remain closed until a slower wheel speed on one wheel compared to others is detected, which is an indication of an impending wheel lock-up. At that point the ABS valve opens and exhausts air from the brake chamber to prevent the wheel from locking and thus skidding.

Generally when brake system air pressure is lost it tends to be a relay valve, but there are other parts of each braking system that can also cause the loss of air pressure. When the coach is not running generally speaking there is no air pressure going to the brake chambers because it is stopped at the relay valves.

A DOT brake check is a good source of clues as to where systems can be leaking.

[Jon]

Thanks Jon for your reply, If air is leaking out of the discharge port of the abs valve while airing up the tanks ( with shop compressor ) to me it implies the relay valve is leaking and carrying over to the abs valve…but it is only on the right side abs valve, would the left not leak as well..? The relay valve mounted on now is the second relay valve I’ve install and I was thinking surely two of these could not be bad ( both are new not original Bendix but reman. ) I guess I will order one from Prevost and see how that goes.

[Archived]

I have to wonder if it is possible to have not only two relay valve failures, and simultaneously have an ABS valve leak as well. The only relay valve leaks I have personally seen were air leaks to atmosphere. If it leaks into the brake chamber, and the chamber is leak free at some point you are going to have a very hot dragging brake and possibly a fire.

[Bernie]

Jim, The more I think about your description of the leaks the less I know. I alluded to it above, but what follows is my thinking aloud. I am hoping some of the other folks on here will jump in because I admit to being troubled by this.

The ABS valve cannot release air if there is no air to release. The ABS valve sits between the air tank, its relay valve and the brake chamber. With the coach parked, the emergency brake engaged no air should be downstream from the relay valve to any brake chamber.

A relay valve does not supply air pressure to only one side brake chamber. It serves to supply air pressure to an axle or axles.

That suggests if it leaks out of an ABS valve air pressure is going to all brake chambers on that primary or secondary system. That would mean the brakes are being applied, except on the side and axle where air is leaking from the ABS valve.

I have no clue if this will help tell you anything, but there is a test of the ABS system every time you turn the key on and are waiting for the ECM lights to go out before starting. Turn the key on, but not to start. You should hear four distinct clicking sounds then silence. What is happening is the ABS system doing a self check. If you want to hear it actually function, air up the coach and shut it off. Apply the service brake and turn on the key. Instead of clicks you should hear four distinct air releases. This is called a “huff” test. After the huff test sequence there should be no additional sounds of leaking or losing air. A partner can assist by helping you determine if during the huff test one of the ABS modules has a different sound from the other three.

I will continue to try to think this through, but if instead of a relay valve leaking to atmosphere it is leaking to the brake chambers I think driving the coach could be a problem. As mentioned previously when the relay valve fails it typically loses air to atmosphere and usually escapes such an amount the bus air system cannot be fully pressurized,

Something isn’t right, but I can’t grasp it right now beyond what I have said.

[Art]

I’ve found the source of the air supply causing the leak, the inversion valve delivery port which feeds the tag axle relay valve signal port . appears to be back feeding the abs valve. Now does that mean the relay valve is bad by bleeding thru to the supply port or the plumbing from the norgrens is wrong…?

[Art]

After reading about the TR2 Inversion being normally open there should be air at the delivery port when airing up, so that leads me to believe the relay valve is bad. I’ll rebuild it or replace it once I open it up and see what it looks like inside.

[Bernie]

FWIW, I have always tried to use Bendix or Prevost OEM parts such as Haldex or other brands Prevost uses. I am not saying other brands or offshore manufactured parts are bad, but I just feel more comfortable getting exact replacements.

Relay valve failures are not very common, but when they fail they are usually open and the bus is not going very far until the valve is replaced or repaired.

[Jon]

Thanks Jon, when I first started ordering parts I thought I was ordering Bendix oem parts only to find out they were either Bendix “style” or reman. I learned the hard way on that now I make sure that anything I order now is Bendix OEM. I’ve been working for over a year now to get where the air tanks will hold air and not leak down, I fell like Iam getting close but not there yet. I finally purchased a ultrasonic leak detector and it helps alot finding leaks especially the small ones.

[Archived]

10-4 on the leak detector. Generally if the drain valves and the relay valves are good the brake system tanks hold air pressure for months. But for reasons unknown I have lost all air over night, likely a relay valve that didn’t seat properly, and then the system would be back to being air tight.

It is when the air loss is all the time that the system is due for some work.