Jizzo17

Yellow circle is the double check. Blue line feeds back to the relay valve which is circled in red. Red arrow feeds all the way to the cab through the firewall into the trailer emergency valve in the dash. Which is the one that is leaking out of the side when I push in the parking brake valve. But does not leak when I push the tractor park brake valve in.

So that relay, that is currently being fed with air from the reservoir, should be sending pressure to that double check valve?

Okay thanks. I also meant to add that I’m not getting air from the relay valve to that supposed double check valve. But I am getting air fed to the relay valve and my rear single canisters work fine with foot brake.

I had some time to take pictures of the two valves I was referring to. I hope this pictures stay in order to avoid confusion… In the first picture, line on the left side runs back up to the red handled valve in the dash which I assume is trailer park. Line on the right runs back to the valve opposite side which is what is in the second picture . In the second photo, 3/8” line on the bottom towards the viewer closest to the driveline is where the right airline from picture 1 goes to.

Thanks for the replies everyone. I have had time to mess with my truck some more. I recently replaced the QR1 valve in front of the front drive axle and traced the parking side up to a T valve on the inside of the frame rail passenger side. It has one line that runs up to the engine bay and into the firewall where it connects to the hand emergency valve on the dash. The other side runs across the crossmember to a bendix valve that is for the service brakes I believe. It is receiving air from a half inch line but pushing the valves in does not direct flow to the “T” valve on the passenger side. I have also run into an issue when running the new wiring to the tail lights. When the key is in the off position and I have air in the tanks I can press the brake pedal and my tail lights work. But if the key is in the on position my brake lights stay on indefinitely. I tried pushing the tractor brake to release them but they remain on. I tried flipping the wires on the air solenoid attached to the tractor park valve but that didn’t resolve that issue. I have also noticed an excessive amount of oil in the air lines which I read could be cause from the compressor. I have installed a new air dryer on it so that should help. I haven’t been able to use it enough to check if the lines are getting cleared out if it would help I can take some pictures when I get to work on it some more.

The aux valve is the valve on the right. Left side red, middle yellow, right black. It has a black knob and says auxiliary on the face. Although the new valve I put in it has a smaller rod diameter and the black (aux) knob won’t fit on it now. So the previous owner may have put a different valve in it. I'm not dropping any noticeable pressure when engaging the yellow valve. I can only get it to stay engaged once I get the pressure up to 90. I have my governor set to 120 and the air dryer pops off accordingly.

Hi everyone, I recently purchased a 1985 Mack R600ST dump truck (S/N 1M1N179YXFA099577) on auction and i have having trouble figuring out what is wrong with the tractor park brake. The issue is it is not getting air to the brake chambers and the previous owner had pulled the slack adjusters to where there is no engagement by the spring for the truck to be in park. I can get them to move with my front brake and I have brakes to my rear drives adjusted properly. The tractor valve will only push in and hold at 90psi. If I were to push in the aux valve at the same time it would pop the tractor valve back out. So I replaced the aux valve and the tractor valve now will hold if I press the aux valve in. I do have a small leak in the side of the trailer supply valve and it was going to be my next valve to replace. I have replaced the tractor park hand valve, the aux hand valve, and the double check valve all with OEM mack parts. I have also replaced one of the 30/30 brake chambers that I was led to believe was feeding back into the system. Any input would be greatly appreciated and thank you in advance.