I've got a 94 RD 688S log truck. Something's wrong with the air system. Air pressure builds to 120 in both primary and secondary tanks, but parking brake control button won't push in/release. Air horn and shifter splitter also not getting air. When I drain the air pressure in the secondary tank to less than 30 psi whatever blockage and/or malfunctioning valve "fixes" itself and parking brake controls, air horn, and shifter splitter all commence working. Once secondary pressure builds back up 90psi or so, setting the parking brake just once causes the blockage and/or malfunctioning valve to quit working. No more air horn, no more releasing parking brakes, no splitter shifter, etc... After a day and a half of head scratching, I think I've found a valve on the driver's side (in the vicinity of the quick release valve) that has both secondary and primary air connected to it, with a diaphram held in place with a ring-clip sort of thing. Can anybody tell me what this valve is called? Can anybody suggest whether this might be the answer to my troubles? Any other suggestions? My theory is that it has been cold. I left the truck parked facing uphill (a pretty good hill) for about a week or so in cold weather. The slope caused condensation to run down the line and build up behind the diaphram and then when it froze it pushed that ring-clip out causing the valve to malfunction. But I can come up with a theory for just about anything, except maybe why I would quit a perfectly good job to go into logging