Big Air leak when on brake

[eric787]

Got a 07 Mack chu613 pinnacle with aggressive air leak when brakes applied. Its dumping out a little yellow air line that exhust out on the fire wall. Only leaks when applied. Replaced brake pedal valve and a stick valve in the rear of truck. I'm at a loss, its hard to diagnose while trying to hold brake with a stick. I have 20 seconds to run around and look at stuff before its out of air.

[Geoff Weeks]

Trace the "Little yellow line"back to what it is connected to. You know where the air is exiting the system, so work back up until you find the problem.

Does it drain both primary and secondary tanks? That would be a clue it is a problem in the trailer portion (trailer brakes can be fed by either primary or secondary so a failure in one does stop the trailer from braking). There are portions of that system that are "live" even if no trailer is attached or the "red button" is pushed.

[Geoff Weeks]

Does it leak when the "Johnny bar" (Trolley valve) is applied?

[eric787]

there is no trolley brake but yes both tanks drop and the hose goes into that little manifold on the firewall

 

[eric787]

sorry my mistake it does have trolley brake but that hose doesnt leak when i use that.

[Geoff Weeks]

wow, I stopped working on trucks commercially in the very early 2000's and it was very rare to see a truck without a 'Johnny bar" even if it was a straight truck (dump truck etc).

So, because there are two complete systems that are supposed to be isolated from each other, to prevent a failure on one system from effecting the other, you should start looking where the two come together to feed the trailer brakes.

This would include the double check for the tractor protection valve, the double check for the Johnny bar if equipped, and the tractor protection valve it self if it is one of the newer type that has the double check inside the tractor protection itself, unlike the old TP-3

Note, the tractor protection valve is NOT the red and yellow buttons, that some call the tractor protection valve.

[Geoff Weeks]

Ok, we were both writing at the same time, but that is helpful. That should rule out the Tractor protection valve, as it should see application pressure when either the foot or hand valve is used.

[Geoff Weeks]

My 1st guess would be the shuttle check that separates the Johnny Bar from the foot brake feed to the tractor protection/trailer feed.

[eric787]

where would that be located? so it turns out that the hoes that does exhaust when applying break runs threw firewall from motor side right to the trolley lever.

[Geoff Weeks]

There should be 3 lines to the Trolley, (2 on earlier systems where they just exhausted the air into the cab) One should be supply pressure and one should be output that will go to the double check. 3rd is the exhaust you traced.

Where it would be located is up to the mfg and I have no idea on that model, where they put it. It could be located anywhere between the trolley, foot valve, and the tractor protection valve. Some are in the cab, some are down on the frame.

If you are in a pinch and need to move the truck, remove and plug the output line from the trolley valve (plug the line, not the port on the trolley valve) and the leak should go away if the shuttle (double) check is the problem. You should still put the time in to fix it right.

Edited Friday at 03:18 PM by Geoff Weeks

[eric787]

ok great ill give that a try. truck moves fine just cant hit brakes more then twice in a row or it emptys. its in shop now so i can mess around with it. there is 3 lines should i trace the input or output and see if it leads me to that valve

[Geoff Weeks]

Output from the trolley.  It goes to a T valve where the higher of the two inputs goes to the output (double check). Right now it is not sealing so air can come from the foot brake, back feed up the line to the trolley and since in its normal position that is open to exhaust, the air flows out the exhaust.

 Another way to test it is to build full air pressure, fully apply the trolley, then step of the foot pedal. It shouldn't leak until you release the trolley valve.

Input to the trolley should have system air, output should be 0 psi with the valve in the normal position, and of course the line you already traced just dumps to under the hood. 

With the air built up, if the line you crack loose at the trolley has pressure, that is not the one you want, if you crack it loose and it doesn't have pressure, then remove it and plug it, then test the foot brake to confirm the problem has gone away. then trace that line to a valve with three lines on it.

[eric787]

so with trolley brake applied then applying brake pedal no leak out that exhaust hose. traced the output from the trolley back to this valve with 2 hoses goin to it. pink ziptie is the output i traced. wondering if this valve looks correct.

[eric787]

so with trolley brake applied then applying brake pedal no leak out that exhaust hose. traced the output from the trolley back to this valve with 2 hoses goin to it. pink ziptie is the output i traced. im wondierng if that is the valve. it only has 2 lines goin to it. idk what happend with the post with the pictures

[eric787]

and now for some reason after disconnecting that line hitting the trolley brake to confirm that was the line. connected it back together now everything works as it should for some odd reason???

[Geoff Weeks]

Yeap, that is the double check, the outlet is the pipe connection that looks like it connects to a tractor protection valve.

The reason it fixed it is because without all the air line connected the flow out the exhaust was higher and that was enough to get the shuttle to move. It sound like it was "sticky". It may work for a long time or it may fail shortly, but at least you know where it is and how to fix it.

Doesn't look like fun getting it out of there, however.

[Geoff Weeks]

Make sure the line between the double check and trolley valve is not "crimped" anywhere along its run.

[eric787]

thanks for all the help i really appreciate it!

[Geoff Weeks]

Glad I could be of help.