Not getting air to parking brake controls, air horn, splitter

[raindog]

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

[gearhead204]

wish I had one handy to look at. if memory serves you have three tanks wet, pri,sec, and pri and sec are the same tank.on one end there is a pressure protection valve its about 2in. square and has two round thimble sized chambers on the bottom half of this valve. this valve allows air to travel both ways above 70 or 90 psi (don't remember 4 sure) but will work as a check valve above or below set pressure you might check this as its not too hard to get to. the other valve you speak of I believe is shifted once you make a application on the treddle valve over 60 psi and combines both pri. and sec. air for emergency stop. a space heater and a tarp plus some air line anti freeze might be a faster fix!

[raindog]

Below is a picture of the offending double check valve. It is mounted on a crossmember, driver's side in the vicinity of the quick release valve. The replacement part recommended is the DC4 double check valve, which looks a little different from the original part (below) but it functions the same and is probably a bit sturdier than the old one. Hope this will help someone who may have a similar trouble down the road...

[Johnny Rock]

I know this post is five years old. I just had the same exact problem on my 1994 Mack RD. After two days replacing parking brake valve on dash, relay valve on frame and troubleshooting, no results. I had no choice to search the internet. I am so lucky to of found this posted long ago. I am done replacing that double check valve and everything is back to working properly. I want to take the time now to thank Raindog.

Thank you Raindog for sharing ! ....You the Man !