AIR COMPRESSOR FAILURE
The purpose of this article is to relate to other owners the symptoms that accompanied the failure of my air compressor.
The air compressor is a very important mechanical part of any vehicle chassis that has air ride and air brakes.   It is the compressor that inflates your air bags and also operates your brakes.   You should never drive your D without the air bags inflated or you will cause very serious damage.   Without air, your rear spring brakes will be applied and you will not be able to go anywhere.   If you lose your brake air pressure while driving, your back wheels will lock up and you will very quickly come to a stop.
Here is a brief description of our saga.   I present it here primarily because of the confusion is caused in discussing the problem with FCOC in Gaffney - confusion which could have been catastrophic.
Our problems began one cool fall morning in Deming, NM, when we started up the D at one of our favorite boondock sites.   For no apparent reason, the secondary air gauge would not come up.   This is the air circuit that inflates the air bags, so it was obvious we were not going anywhere.   We did not even attempt to move since we knew about the serious damage that could be done without the air bags inflated.   However, had we attempted to move, we would have quickly depleted the air in the primary tank also, and this is the tank that provides primary air to the air brakes.   Overnight, the secondary tank had fallen to approx 60 psi, which it never did before in just one night.
So, I made a call to Freightliner in Gaffney (1-800-FTLHELP).   Making a diagnosis over the phone is difficult at best.   Since my coach had only 45,000 miles on it, they suspected that the problem was a faulty ride height control valve - and not the compressor.   They suggested that I tap the ride height valves lightly with a hammer and also raise and lower the coach several times.   After about 15 attempts, all of a sudden the secondary air gauge came up, along with the air bags.   FCOC was not pretty sure that the problem was a ride height control valve and assured me that, once unstuck, it would not fail while going down the road.   They also said that this was likely just a "fluke", so I was not concerned about having it inspected immediately.
All was well through our day of travel - and we spent another night boondocking at a picnic area in Texas.   Well, the next morning, we had the same problem.   This time, it took a lot longer (raising and lowering the jacks and tapping on the ride height valves) to get the air to come up.   We were many miles away from a Freightliner facility, even though we had now resolved to get this into the shop as soon as possible.   Again, we drove all day without a problem.   We had planned to make it to San Antonio Freightliner that day, but the terrible storms stopped us about 60 miles from our destination.   So, tonight would be spent in another picnic area, but tonight we would not deflate the airbags.
Alas, even though we did not deploy the jacks and did not deflate the air bags, not only was our secondary tank showing about 60 psi, but now the primary tank was showing 60 psi also.   This time, no amount of raising and lowering the coach on the jacks would work.   It was also raining and I was not about to try tapping on the ride height valves.   So, we ended up having to be towed 60 miles to the Freightliner facility near Converse, TX.
By fanning the brake, I could deplete the air.   However, the compressor would slowly build air pressure back up to near 60 psi, but it could not build it up much beyond 60 psi.   Also, any application of the brake pedal resulted in immediate drastic loss of air.
I do not know how the air compressor operates.   I had assumed that it either failed completely or that everything was fine.   I guess that the folks at FCOC assumed the same thing also, coupled with my relatively low miles.   I had talked to several techs at Gaffney over the almost three days we had a problem.   Steve Horton was going to have me do some detailed troubleshooting on the compressor, but alas my cellphone battery failed.   I now carry a second battery!
The bottom line is this - if you notice ANYTHING abnormal with your air pressure system, get it checked out at a qualified service facility immediately!