Air system leak

[itdave]

While driving I see the gauge for the rear air tank dropping from 125# to 90# in less than a minute then the compressor kicks in to bring the pressure back to 125#. The front air tank gauge holds at 125# until I apply the brakes then it moves down gradually in sync with brake use. The problem with the rear tank does not start until I have been driving for a while and seem to happen more on freeway than surface roads. My brakes are working fine and there are no air leaks when parked. Does anyone have any idea what may be happening or how to troubleshoot a problem that only happens while driving.

[moisheh]

Dave: I thought by now someone would have jumped in. I am no expert on air systems but could it be one of your levelling valves? They often are the cause of leaks. When you are parked for say 6 hours does the rear air go down? Maybe it is the "spit valve" on your air dryer?

[JManatee]

it seems odd to me that your front would stay right at 125....

Both my front and rear go up and down at about the same amount while driving... I assume because of the airbags and braking.

then the compressor kicks in and they both pump back up.

[Mike Thomas]

Sounds like your brakes are leaking. You will have to apply the brakes while someone looks for the leak. If you can find someone with an ultrasonic leak detector you will find it pretty quick. HVAC guys will probably have one to find leaks in air conditioning and refrigerant systems. I have used it very sucessfully on our semi trucks and trailers.

[moisheh]

Dave: There is a test you can do to check the air brake system for leaks. Build up your air. Shut off engine and apply foot brake. Hold for 60 seconds while watching the air gauge. Pressure should hold within 3 psi of your cut off pressure. When parked can you hear any air leaks? I still suspect a levelling valve. Easy to change other than safely getting under the unit.

[itdave]

Thanks for all the replys so far.
JManatee the front air tank does operate as you describe.
Moisheh and others regarding brakes leaks, I had the whole brake system checked at the Sacramento Truck Center (they were recommended by DeMartini). They found that the cams had not been lubed and other components need lubrication. They found that shoes and drums were fine and there were no air leaks. Once they completed the work the brakes passed all test.
The air leak continued following the brake inspection and repair.
When we got the coach out of storage before starting this trip we found that the rear passenger side air bag was deflated while all the others were at ride height. So I agree it may be a ride height valve.
Does anyone know how to replace the ride height valve?

[Rex]

Dave, were both air bags on that side deflated? Air the coach up and use a soapy solution to check the air valve and fittings on or near the air bags for leaks. Also wet the air bags down with lots of soapy solution and check them for leaks. I had a very slow leak on my front drivers side and it turned out to be an air bag, in fact both of them. There were very small air bubbles even with the air bags at full pressure. A leak at an air line fitting or the ride height valve will be more noticeable.

If the problem is the ride height valve, changing it is no big deal. Setting the ride height is tedious. For your 06 Endeavor, the numbers are 8 13/16" front ; 9 1/16" rear.

To release the air lines from fittings, use an open end wrench that just fits over the air line and press HARD against the fitting while wiggling and pulling the air line away from the fitting. To attach the air line to the fitting, just press it in firmly.

Not sure about your coach, but the ride height valve may be Haldex-Midland Air Height Control Leveling Valve 9005407. You will have two of them on the rear, one for each side. Only one on the front axle.

[itdave]

Thanks Rex
As usual you provide great suggestions. I think I will replace the rear passenger ride height valve and see of that fixes the problem.

[slickest1]

Another possibility could be an airline to the park brake or one of the air brake chambers. The way to check this is to build your air up to max. shut of engine and block your wheels so it won,t roll away and release your park brake.If that is the problem you will be able to hear it then. When you are driving the park brake needs air to keep it released and when you stop and engage the park brake it exhausts the air. They would get there air supply from the rear tank. If you are working on air levelers be vey cautious, when airbags deflate the coach will come down and if not supported properly could crush you. Good luck.