Air Brake Info and Troubleshooting

4XDesign · Jan 5, 2018

Oxyacetylene said:
The park brake dash valve is a Haldex KN20035. The trailer dash valve is a Haldex KN20025. On these first generation trucks those two valves are separate. On newer trucks they are integrated into a single valve assembly which is also serviceable. Maybe one day I will swap out the two individual valves for this single valve setup, I like the idea of being able to service it.

I have been reading through all of your posts in this thread. I have actually replaced all three of the valves behind the driver step cleaned the one way check valve behind the lower bumper grill and gotten the air leak to the emergency glad hand to stop. In the above quote you referenced the part number for the parking brake valve in the cab have you replaced this valve? Reason I am asking is because I cannot release the brakes on my truck if you attempt to depress the valve it springs back at you like it is pressurized also the truck will not build air and is exhausting all of the pressure in the transmission tunnel in a vent that I am guessing vents air from the parking brake valve. I have also taken the other valve off that is located under the same lower bumper grill. I am going to replace it as well because it appears to be bad air comes out of all the ports no matter which port you put air into it is plastic and not serviceable but I assume this means it is bad. If you have any more insight on my troubles just please let me know and thank you so much for all of the great info you posted.

Oxyacetylene · Jan 5, 2018

4XDesign said:
I have been reading through all of your posts in this thread. I have actually replaced all three of the valves behind the driver step cleaned the one way check valve behind the lower bumper grill and gotten the air leak to the emergency glad hand to stop. In the above quote you referenced the part number for the parking brake valve in the cab have you replaced this valve? Reason I am asking is because I cannot release the brakes on my truck if you attempt to depress the valve it springs back at you like it is pressurized also the truck will not build air and is exhausting all of the pressure in the transmission tunnel in a vent that I am guessing vents air from the parking brake valve. I have also taken the other valve off that is located under the same lower bumper grill. I am going to replace it as well because it appears to be bad air comes out of all the ports no matter which port you put air into it is plastic and not serviceable but I assume this means it is bad. If you have any more insight on my troubles just please let me know and thank you so much for all of the great info you posted.

The park valve will not stay depressed unless you have adequate air pressure. It works as a fail safe in that regard. Pressure drops and the valve pops. The valve might be fine and the problem just that you can't build enough pressure in the system. How much pressure do the front and rear brake gauges come up to? You need to find the source of the venting air and work backwards from there. If you have an air compressor you can make an adapter and pressurize the system from the front emergency gladhand, but make sure you chock the wheels first! This way you can listen for leaks without the engine running.

I haven't replaced the valve on mine yet. Mine is fine for now, but when it needs replacing I plan to replace both valves with the one piece body version that has both valves in it. There is another thread on here about making that swap. The one piece body version is serviceable, from the top, so you could replace seals inside it without taking the valve out. The air lines on my valves are a little short, so I have been putting off the conversion just because there isn't much slack to work with.

4XDesign · Jan 5, 2018

The problem first arose as I was backing the truck into its parking spot, something failed and air blew off like if you open the valve on an air tank or something the truck lost all pressure and brakes locked. The air is escaping from a vent that comes out of the bottom of the cab. If you were to reach your hand up behind the grill and touch the bottom of the cab directly above the gladhands. I am going to have to pull the heater assembly out-of-the-way to see where the other side of this vent goes. I have plugged the vent in order to get the truck to build air pressure it will build pressure to cancel the dash lights 90-100 psi but you can't depress the park valve. I guess I originally asked the question since you had the part number, I thought you may of had trouble with that parking valve as well. I have a trade work out for this particular truck so I am just working to figure out all the bugs.

Suprman · Jan 5, 2018

I would start with the 3 valves under the drivers side by the steering box. If more than one is bad you can get all kinds of symptoms. Don't plug vents. It never works out well.

Suprman · Jan 5, 2018

I see your above post where you say you replaced them. There are a lot of valves on the truck. For this issue I would follow the tm troubleshooting flow charts and see where it takes you. Important that no gladhands are blocked off. Will cause all kinds of issues. Could be a treadle valve issue. The tms can be downloaded at the top of the forum.

coachgeo · Jan 5, 2018

Suprman said:
... For this issue I would follow the tm troubleshooting flow charts and see where it takes you. .... The tms can be downloaded at the top of the forum.

For clarity; They are not at the top of the FMTV Forum. They are in the Technical Manuals Forum. All the forums are reached via the top page of this Bulletin Board.

Oxyacetylene · Jan 5, 2018

4XDesign said:
The problem first arose as I was backing the truck into its parking spot, something failed and air blew off like if you open the valve on an air tank or something the truck lost all pressure and brakes locked. The air is escaping from a vent that comes out of the bottom of the cab. If you were to reach your hand up behind the grill and touch the bottom of the cab directly above the gladhands. I am going to have to pull the heater assembly out-of-the-way to see where the other side of this vent goes. I have plugged the vent in order to get the truck to build air pressure it will build pressure to cancel the dash lights 90-100 psi but you can't depress the park valve. I guess I originally asked the question since you had the part number, I thought you may of had trouble with that parking valve as well. I have a trade work out for this particular truck so I am just working to figure out all the bugs.

I did have a leak from that same vent, but it wasn't a huge leak. Mine turned out to be one of those 3 two-way check valves behind the driver's side step. If you have already replaced yours it could be something else. How long ago did you replace them? It could be worth a sanity check to take them loose and check the diaphragms if all else fails.

Suprman · Jan 5, 2018

They are in the top section of the main forum page.

4XDesign · Jan 5, 2018

Oxyacetylene said:
I did have a leak from that same vent, but it wasn't a huge leak. Mine turned out to be one of those 3 two-way check valves behind the driver's side step. If you have already replaced yours it could be something else. How long ago did you replace them? It could be worth a sanity check to take them loose and check the diaphragms if all else fails.

I replaced all three with new this morning. In doing so it did fix the leak at the emergency gladhand out front along with cleaning the one-way valve close to the gladhand. I will check out the flowcharts to see if that will lead me in the right direction.Whatever the culprit turns out to be, when it failed it dumped 100 percent of the air of the truck is making out of the vent under the cab. I will report back my findings. Thanks again for all the help.

Suprman · Jan 5, 2018

Maybe something else wrong and you burst a valve diaphragm in one or more of your new valves.

tennmogger · Jan 6, 2018

The brake treadle valve (foot pedal) can get dirt in it, leak out that vent you describe up under the center grill, and dump air to zero.

There's a thread on that problem. I will try to fine it unless someone beats me to it.

Edit:. This thread is good:

https://www.steelsoldiers.com/showt...blems&p=1900593&highlight=Treadle#post1900593

4XDesign · Jan 6, 2018

I worked on the truck for a while today. I used an air hose to air up truck and I pressed the brake pedal repeatedly, I actually got the leak to stop. After this it was pretty apparent it was the treadle valve, so I disassembled it in place and cleaned it. There was some grit in it, I used a little pb air tool lube when reassembling. I cranked the truck up after reassembling and it aired up fine and the brakes unlocked as well. I think there still may be a little bit of contamination in the valve because there is a slight leak occasionally but for the most part it is completely leak free.
Thanks again for all the help

alleuro · Feb 28, 2018

Suprman said:
The front gladhands are for towing only not supply like on the M939 trucks. If you look on the cover it has a vent on each same as axle vents. If the vents are clogged or if some bug nests in there and it cant breathe, this can cause the brakes to not release. Behind the front grill is a small valve this also can go bad and not allow the brakes to release.

I just found this thread and I think I figured my problem out I have a bunch of ice on the front of my truck and the front brakes locked up. Thanks one more lesson learned!

KGBruce · Apr 16, 2018

This thread has been incredibly helpful for tracking my air leak. My problem is I have no way to build air up in my brake system to diagnose my air leak. So far I have just applied air into the rear service glad hand and followed the air leak to the front vent under the cab described here as the vent for the treadle valve and park brake. I'm not sure if this is giving me a false symptom doing it this way. I would rather be able to charge the air holding tank and diagnose from there . Is there anyway to do this? Am I doing this incorrectly by using the rear service glad hand to pressurize the brake system?

Suprman · Apr 16, 2018

You can charge your air system thru the front red gland hand but you will release your spring brakes at the same time to the truck has to be chocked. You can remove one of the drain valves on an air tank and screw in an air fitting and fill it that way also.

KGBruce · Apr 16, 2018

Thanks Suprman! Will definitely do that next! Truck is chocked and ready!

Duckworthe · Apr 26, 2018

Just stopped by my local truck shop yesterday to get parts and figured I would post these pics up so others can see what the inside of an airbrake chamber basically looks like. Just for your curiosity.

Reworked LMTV · Apr 27, 2018

I see the blue spring of death.

19Detail · Jun 1, 2018

I had an issue with my Front and Rear air gauges not reading correctly (they maxed out as soon as power was applied to them, reading 120+ psi all the time). The issue turned out to be a bad wire from the gauge to the transducer(?) below the dash. All the wires were connected, but the right wire on the left transducer was actually broken inside the wire insulation. It looked connected but it wasn't. I found that out by bumping the wire while air was building and the alarm came on. I cut off the connector and put on a new one and it works. That one wire took out both gauges.

Jason
1995 LMTV

ramboflyer · Jun 13, 2018

I have that same vehicle and for some reason the parking breaks when released at the dash will not release on the wheels. I suspect the cab tilt switch since that was the only thing raised prior to the break problem. I noticed the little red pin in the middle of it doesn't always retract back in and I've tapped on it and then ut sucks back in. any Ideas ????

Air Brake Info and Troubleshooting

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