Air brake issue

[Jizzo17]

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. 

[Lmackattack]

what is the aux valve you speak of?

 

Most macks had 3 buttons

blue= tractor only 

yellow= parking (both trailer and tractor)

red= trailer air supply

 

If you are dropping to 90psi when you push in a button Id check the tractor protection valve or the brake buttons first (Primarily the yellow and red buttons)

 

 

 

 

[Jizzo17]

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. 

[Freightrain]

Most brake valves won't release until you have a specific air pressure.  Mine is like 70#.

[Brocky]

1 hour ago, Freightrain said:

Most brake valves won't release until you have a specific air pressure.  Mine is like 70#.

And they must pop out at automatically and set the parking brakes at a set low pressure value.. If I remember correctly it is about 60 PSI ??????? Most roadside "Diesel Cop" inspections the cop will remove the emergency line from the trailer and record the PSI that the button pops.. If it is too low you are grounded!!

[Lmackattack]

the air pressure needs to be above 60 psi to be pushed in without it trying to pop out. I think the actual pop out psi is closer to 40 psi but I always wait until 60psi to push them in

 

that aux valve needs to be traced to what its for. Like I said most R models had a blue vlave for tractor parking. If the yellow is pushed in first you had to make sure you pushed in the blue as well.

 

In this photo the 3 valves from left to right..... trailer supply......truck trailer parking........tractor only park

[Joey Mack]

I like the twin sticks, and the revolver cylinder knob..

[h67st]

Have you disconnected the QR1 valve before the maxibrakes to see if the line feeding it gets air when you push the tractor parking valve? If it doesn't, trace the line back from there, maybe a section of the line is crushed or collapsed on the inside. Hopefully someone didn't work on the truck before and hook something up wrong.