Setting parking brake dumps all air?

[Brumini]

My 1995 Monaco Dynasty dumps all of the air in the system when set. Then it will not build air again unless I hold the parking brake button down hard until it is over 20 psi. After that it fills to 120 psi like normal until I set the parking brake again. I can hear air under it in front of the rear axle coming from opening on the valve shown. It looks like that is a standard place where air is released when needed but the parking brake should not drain the entire system. Does anyone know where I should start checking things? 

[RoadTripper2084]

Hi, do you have schematics for your year for the air system?

Here are diagrams from my '97's system, probably similar to yours.

 

From your description of the problem, I think you should start by looking at the relay that disengages the spring (park) brakes on the rear drive wheels. Highlighted in orange below:

My understanding is that the air pressure held by the park brake switch/valve in your console acts as a signal to the relay to dump the air (engage the park brakes) or pressurize with air.  It seems like when it dumps the air from the rear parking brake cans, the relay (above) that controls whether air flows into the springs brakes or not is sticking and letting all the air dump into (and out of) the spring brakes. Since these are critical safety item, both air tanks will feed into those brakes.

So find that relay and remove it and disassemble to see what you've got. I successfully removed a similar relay on my rig last fall and was able to clean it up and re-lube the o-rings and it's 100% now.