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.

[Jizzo17]

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. 

Edited Thursday at 02:40 AM by Jizzo17
Added some info

[cruiseliner64]

Yes more pictures pleade

Paul

[Adkat]

Have a look at this, should help make sense of how everything should be hooked upBW1974_US_005.pdf

[h67st]

5 hours ago, Adkat said:

Have a look at this, should help make sense of how everything should be hooked up.

That's a great chart, very handy to have around!

[Geoff Weeks]

Going way back in my fuzzy memory, some trucks with the ign on and the parking brake set, would activate brake lights. With key off brake light switch is fed from another switch that is always powered.

Not 100% 'cause we are talking about a 1/2 century ago.

Get the air system fixed 1st then see where the electrical is at.

Air systems are not all the same, and can vary widely. Some use relay valves on the spring brakes some do not, some have anti-compounding on the spring brakes, others do not. some use inversion valves on the spring brake side for emergency use of the spring brake when half of a dual air system is down.

That is why it is so important to have pictures and label where each line goes.

 My 9670's didn't have any relay valves on the tractor, anywhere! straight piping from the treadle valve to the Q/R on the axle housing itself. 

[cruiseliner64]

Our Cruiseliner wiil put on the brake lights if the ign is on and trailer brake is applied...even just a tiny bit applied.Yes I found this the hard way,spent hours trying to find out why brake lights wouldnt go off only to discover the trailer brake was applied just a touch........🥶

Paul

Edited Thursday at 08:59 PM by cruiseliner64

[T-Mack1]

Low pressure warning should come on at 60 psig,  The valve should pop between 45 and 30 psig.    The way you have a tiny bit of tine to get off the road before the brakes lock up.    

 

[Jizzo17]

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.

Edited Saturday at 01:56 PM by Jizzo17

[Geoff Weeks]

last picture looks like a double check

The other is a relay valve.

[Jizzo17]

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. 

[Geoff Weeks]

 

Double check passes which ever supply is higher to the outlet, without allowing the higher inlet pressure to pass to the lower (or no) pressure inlet port.

The thing with 5 lines going to it is a relay. the two lines  closest to the frame, parallel to the frame should be reservoir pressure, the two perpendicular to the frame are brake pressure output and the one on top is signal air pressure.

[Jizzo17]

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

[Geoff Weeks]

Draw a line diagram of how the air lines are run and to which ports and post here so we are all on the same page as to what we are talking about.

[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. 

[Lmackattack]

On 1/7/2025 at 3:39 AM, Joey Mack said:

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

I think the revolver knob was given to me by Rob Swallows (BMT member "Rob") Its been on the truck for 20 years but I have a hard time remembering yesterday

 

[Lmackattack]

sounds like its backfeeding the air. If the check valve is faulty that can be a reason and likely a cheap part to throw at. Im not very good with air brake systems and on a old truck who knows whats been changed.  

 

like wise you can test the check valve to see if air flows in the correct direction before replacing it but it might just be smart to replace it simply because of its age

 

Just another thought I remembered. I had to replace a dash valve 5 years ago on my R model because it was leaking. the valve just failed because of age   well. 

 

[Geoff Weeks]

What is confusing me, is nothing should go straight to the trailer red knob. The red knob outlet should go to the tractor protection valve (TP-3 or similar) on the frame.

That said, I have not been around a lot of 3 button trucks, and the few that I was, never had a problem.

This is what I think I am seeing in your photo, but you notice it is not connected to the trailer valve