Air Pissing out

[nbone28]

2015 Mack Pinnacle cxu613, when I go to push in the air supply for the trailer (and when i release it) air is pissing out from underneath my hood. I’m not losing air when I go to brake, it’s only when I go to engage or disengage to air supply for the trailer. It’s pissing out of the top yellow airline in the photo. It looks like there is a valve behind the firewall, could that be what’s causing this? Thanks for the input. 

[mechohaulic]

did you try and dead head the lines to trailer then push  the buttons in ?? might help eliminate whether issue is with truck or back feed from trailer. where in CT is this truck located??

[mechohaulic]

yellow lines are the exhaust ports I believe, 2015 way new for me to be certain.

[nbone28]

4 hours ago, mechohaulic said:

did you try and dead head the lines to trailer then push  the buttons in ?? might help eliminate whether issue is with truck or back feed from trailer. where in CT is this truck located??

Trucks over in the New Haven area. It’s only occurring when engaging and releasing the air supply to the trailer. I’m not losing air when I go to brake. 

[mechohaulic]

air is constant leak out the exhaust yellow line when you push in the red button  to release trailer brakes ;; correct??? do you pull same trailer or others??  same we continue ; other trailers being hooked to , does exhaust still leak,?? attempt here is to verify the truck has the issue

[Joey Mack]

Heck, un hook the emergency line on each trailer brake chamber and cap the line. Then release the brakes and check for the leak. Do one at a time, hooking the removed line back up before going to another chamber. If the leak goes away, the the chamber with the line removed has a diaphragm leak. You can put a piggy back on it, or replace the whole chamber..

[Mark T]

This didn't happen to start after that foot valve was changed, did It ?  

[mechohaulic]

3 hours ago, Joey Mack said:

Heck, un hook the emergency line on each trailer brake chamber and cap the line. Then release the brakes and check for the leak. Do one at a time, hooking the removed line back up before going to another chamber. If the leak goes away, the the chamber with the line removed has a diaphragm leak. You can put a piggy back on it, or replace the whole chamber..

i wasn't to successful getting the trailer lines disconnected and dead headed to eliminate trailer i guess. no reply as to one trailer involved or this unit does drop/swap. 

[nbone28]

2 hours ago, Mark T said:

This didn't happen to start after that foot valve was changed, did It ?  

Changed the brake valve out a little over 2 months ago, this just started happening on Monday. 

[nbone28]

4 hours ago, mechohaulic said:

air is constant leak out the exhaust yellow line when you push in the red button  to release trailer brakes ;; correct??? do you pull same trailer or others??  same we continue ; other trailers being hooked to , does exhaust still leak,?? attempt here is to verify the truck has the issue

I pull the same trailer everyday. I stay hooked to it. It isn’t leaking constantly out that yellow line, when you go to push the red button in, air leaks out of that yellow line, the air will drop to 80psi in both tanks, and then build back up. Once it builds back up it doesn’t leak again until releasing the trailer brakes. 

[mechohaulic]

air drops to 80 after pushing in red button then builds to normal indicates trailer loosing air . air loss  would be truck filling trailer air tanks ;; normally. next option would be valve cluster on truck faulty -BUT the air leak would be constant, recommendation would be as earlier stated === remove red/blue lines from trailer; cap off the hoses on truck - number of ways, hook to a hose tender / connect to  another glad hand with plugged off fitting . DO NOT  hook red to blue . with hoses dead headed; push in the red knob . no leak - the trailer has the issue.

[fjh]

so if you hook up the Emerg line and charge it leave the service line off (disconected) is there air feeding back up the trailer service line ????

[Geoff Weeks]

14 hours ago, nbone28 said:

I pull the same trailer everyday. I stay hooked to it. It isn’t leaking constantly out that yellow line, when you go to push the red button in, air leaks out of that yellow line, the air will drop to 80psi in both tanks, and then build back up. Once it builds back up it doesn’t leak again until releasing the trailer brakes. 

Just to be clear:

When you push the buttons in, it leaks ALL THE TIME until 80 psi on the tractor gauges and then stops and builds back up and doesn't leak after that? 

OR;

does it momentarily leaks BUT the pressure drops to 80 and build? 

They are not the same. 

[Geoff Weeks]

I'd be leaning toward the buttons/ MV itself, but without more clarity it is just a SWAG.

Trace that yellow line back to where it goes, that will give important info on what/what system is venting.

[nbone28]

53 minutes ago, Geoff Weeks said:

Just to be clear:

When you push the buttons in, it leaks ALL THE TIME until 80 psi on the tractor gauges and then stops and builds back up and doesn't leak after that? 

OR;

does it momentarily leaks BUT the pressure drops to 80 and build? 

They are not the same. 

When you go to push in the trailer supply you hear the air pissing out from that yellow line outside the firewall. The psi drops to 80 and then will build back up. It holds air perfectly fine after the initial drop. It’ll then piss out air once you go to release that valve. 

[Geoff Weeks]

Again,

Do you hear air leaking the WHOLE time it is dropping to 80 psi, or just a short time while the button is pushed in?

Trace that yellow line back.

As you can see, there are two yellow vent lines going through the firewall so it is important to know what valve this one is attached to so as not to replace the wrong part.

[nbone28]

Just now, Geoff Weeks said:

Again,

Do you hear air leaking the WHOLE time it is dropping to 80 psi, or just a short time while the button is pushed in?

Trace that yellow line back.

As you can see, there are two yellow vent lines going through the firewall so it is important to know what valve this one is attached to so as not to replace the wrong part.

Yes I hear air leaking out that yellow line until it drops to 80 psi, once it rebuilds back up the air doesn’t leak anymore

[nbone28]

2 hours ago, fjh said:

so if you hook up the Emerg line and charge it leave the service line off (disconected) is there air feeding back up the trailer service line ????

Emergency line hooked up still, service line off the trailer, no air coming out the service line. 

[Geoff Weeks]

Looks more and more like the MV (multi-function valve I.E "buttons") are the problem, tracing the yellow line would help to confirm.

[nbone28]

Just now, Geoff Weeks said:

Looks more and more like the MV (multi-function valve I.E "buttons") are the problem, tracing the yellow line would help to confirm.

Tracing that line now, will let you know

[nbone28]

16 minutes ago, Geoff Weeks said:

Looks more and more like the MV (multi-function valve I.E "buttons") are the problem, tracing the yellow line would help to confirm.

Yellow Line is hooked up underneath the orange line on the left

[Geoff Weeks]

Yep, that is the MV valve. Bendix, I don't know (can see from pictures) if it has the cartridge valves that can be replaced separately or you have to change the whole thing. Most change the whole thing because that eliminate a small internal crack in the plastic housing from being the problem and not the cartridge valve itself. 

[Geoff Weeks]

https://www.zoro.com/bendix-module-control-brake-mv-3-028-in-dia-14-npt-800518/i/G111330826/

Here is the valve

If you pull the buttons off and remove the recessed screws you should be able to pull the cartridge(s).

edit:

 the one I linked to was just for generic picture, they come with different ports, some are threaded some a push to connect, get the one that matches what you have.

Should be a number on the Bendix tag I can see on your picture.

Edited 1 hour ago by Geoff Weeks