[Richard]

This incident happened with @"qcj" coach at the rally, and it was a learning experience for us all. 

Here is a picture of the air brake relay valve upside down. I chose this picture because it most clearly shows the problem area. The purpose of this valve is to use air pressure from the brake pedal to open the relay valve. This allows a huge volume of air to press on the brake chambers. When the pedal is released the relay valve opens instantaneously releasing all air on the brake chambers. There is one relay valve per axle, for a total of three on the coach. There may also be one that releases the parking brakes, I am not sure about that. 

First of the all the problem was that the center insert, the moving part of the valve, the white domed part in the picture, failed. This allowed air which feeds the upper part of the valve to bleed excessively. The compressor could not keep up with the bleed rate enough to disengage the parking brake. 

In hindsight, we could have removed the large 3/8 air feed line to the valve and capped it. That would have stopped the leak, and Chappel would have had brakes on the other two axles plus being able to release the emergency brakes. 

Well, we weren't that smart in an emergency and we partially caged the Park brakes and eased him into his parking space. 

What we learned from Tom's bus guy Leo was that the pros simply remove the circlip holding the guts in the valve and replace it with new innards. They do not replace the valve assembly. After attempting to remove the assembly and punting, I understand the approach. The insides are spring loaded so be aware when you take the clip out. Any truck place or NAPA should be able to get the valve or the rebuild kit.