mysterious air leak

islandvagabonds wrote:
OK, we have the description of the tank layout. Why would the front tank pressure cycle only when going down the road?
thanks for the responses
Bob

I would like to point you in the right direction but I am completely stumped by the fact that your leak is only when you are going down the road. :?

Possibly you have an air level valve issue. This would only occur while the suspension is cycling as you drive.

OK, we have the description of the tank layout. Why would the front tank pressure cycle only when going down the road?
thanks for the responses
Bob

I am not sure what you are referencing for your system description but it is incorrect.

The first reservoir after the compressor/air dryer is the "supply" or sometimes called the "wet" tank. From there it feeds each of the two brake reservoirs through check valves. The rear axle brake reservoir is the "primary" reservoir because the rear axle does most of the braking. The front axle reservoir is the "secondary" reservoir.

The tank pressure is not controlled by pop off valves. It is controlled by the air governor. Turns on the compressor when the air falls below about 90 psi. Turns off the compressor when the pressure goes to around 120 PSI.

There is a system relief valve that pops off if there is a malfunction of the system causing it to overpressurize. That is usually around 140 psi.

There is not a front tank and a rear tank. You have a primary tank which is the wet tank and then one or two secondary tanks. The primary (wet) tank should collect any moisture before the air goes on to the other tanks. The primary tank you should be draining the moisture off. It also has a pop off safety valve (automatic air release) on that tank. If it pops off air pressure at less that 140 psi, it should be replaced. Sounds like your is popping off at 120 psi. But someone has to be under there to know for sure. there is a screw out plug on that tank where you can install a air gauge to monitor and see where it is doing. A diesel shop would have a pit to make it easier to get under there and see what is happening!!!
Meritor Troubleshooting Guide

The parking brake is supplied from the front and rear through a double check valve. Theoretically it should draw from the tank with the most pressure in it. Sometime they get stuck and will only draw from one tank.

Thanks to all for the repsonses. The pressures in the front tank cycle only when going down the road. I believe the rear maxi chambers are supplied by the rear tank. When the coach is stopped and foot on the brake pedal pressure is stable in both air tanks.

still puzzled!!
Bob

It could be the air line to that side of the brake chamber. Chock your wheels and release your park brake with engine off and you should be able to hear it and find what is leaking. If it is the chamber just replace the whole unit. Taking one apart is very dangerous, as there is a spring under alot of pressure.

dougmac may be right, it does sound like a ruptured/bad pancake in one of your maxi-brake chambers. The pancake can be replaced, but it is much easier and safer just to replace the whole chamber.

Is the air loss just going down the road or could it be that releasing parking brake is the action that causes it to leak?