WHY DOES SETTING PARK BRAKE EMPTY AIR TANKS?

[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? 

[Frank McElroy]

This looks like an air brake modulator valve that's leaking.  When you set the parking brake it is supposed to release air pressure from the spring parking brakes and prevent tank air pressure from keeping the spring brakes released or in the event of a main air tank failure to modulate the secondary air pressure to operate the drive axle spring brakes when the service brakes are pressed.  I'd remove it and bring it to a large truck parts center and they will match it up with a replacement.