LMTV Parking Brake will Not Disengage

[rjoffrion]

I have an air brake issue maybe you guys can help. I have a M1079A1 that when I push the parking brake button in to release the brakes it vents air out the vent port in the middle of the cab behind the heater unit down to the grill.. I have checked cleaned or replaced all the two way check valves, all the valves in the back on the frame rail, including replace the brake pedal and parking brake button assembly. Im stuck and the truck Is in the bay and wont move.. Any help would be great.

 

[NDT]

Was everything fine and then this started happening, or has it been like this since you got it from Uncle Sam? We see a lot of times that hoses are not connected properly by attempts at maintenance. I would check the connections at the back of the dash valves.

 

[Suprman]

Sounds like a bad air connection or broken line somewhere.

 

[rjoffrion]

well im not sure exactly was the original issue.. a few mechanics have worked on it and couldn't fix it so I got it dumped into my lap.. As I can tell and hooking the lines up from the parts manual they are correct. I also looked at another truck in working order for the brake pedal line routings to the floor board and matched them up. the Schematics that I have don't match up to the vehicle.. I believe it is one of the early A1 since SN is 117612. I have TACOM and PS Magazine working on the issue too.. the top two way check valve ( the one with sensor) was bad/stuck I cleaned it so it stops dumping air out of the glad hand. it just seems like it back feeding through the brake pedal into that vent port.

 

[rjoffrion]

it builds and holds pressure till I push the parking brake button. the trailer one works fine..

 

[Suprman]

Tractor valve with a bad diaphragm can cause air to come out of odd places. I dont have much a1 experience though. What motor does it have? Early A1 has 3126. 3116 would be first gen.

 

[Overdrive]

I have an air brake issue maybe you guys can help. I have a M1079A1 that when I push the parking brake button in to release the brakes it vents air out the vent port in the middle of the cab behind the heater unit down to the grill.. I have checked cleaned or replaced all the two way check valves, all the valves in the back on the frame rail, including replace the brake pedal and parking brake button assembly. Im stuck and the truck Is in the bay and wont move.. Any help would be great.

Do you have the two separate valves for Park/Trailer or the single integrated valve?

Pulled out is "Park Applied" so your venting to the exhaust port when your pushing in "Park Release."

When I switched to the integrated valve setup (the single unit for both valves) I had this issue when I didn't have one of the supply ports providing air to part of the valve. With the integrated unit it still functions the same as the stock two valve setup in that the air chambers are separated from each other....

Check the air lines on both your Park/Trailer valves looking for the supply lines in particular looking for leaks or any weird plugs. I would guess that the Park valve has gone bad if all the lines look good and its failing safe by keeping the park brakes applied.

OD

P.S. I have posted in my dash valve write-up thread that there are only two things that vent out the exhaust port behind the grill and in front of the cab. The park/trailer valves and the brake pedal valves. FYI.

 

[BRob18E]

I found the answer to my issue, I had no working parking breaks, and the CTIS system wasn't working. Then I seen a spark from the electrical box. Once I Pulled it, there was one cable that was lose in the back. Once I fixed this, the parking brake worked as it should and so did the CTIS system. I spent countless hours trying to fix these issues, and it was one lose connection under the panel. It's about 10 minutes to get it off, take the time and see if everything is as it should be, that would be my recommendation.

 

[BRob18E]

All yeah, I also had air coming out of my front glad hand. If you are having that issue as well. I'm assuming that connection was to one of the many valves in the system.

 

[valkrior]

She's roll'n again! Was the forward gladhands afterall. I thought I had cleared this fault early on, but out of desperation I went back to give them a better going-over... Broke them down and cleaned them thoroughly with brushes and then blew out gladhands and lines with compressor. Jumped in the cab, hit the brake valves and heard the springs compress (engine off). Started up, put in gear POP! POP! and away we went. The pops were the shoes releasing from the drums; I did get them wet on my last drive, but though it was weird that they had rusted to the drums in only two weeks.

Now I have a new xmsn problem. Going to start a new thread for that...

Thank you to all who commented on this thread!

Hey, I think I might be having the same problem you had. I jumped in today, and the brakes seem locked up. I don't think they are releasing. Plenty of air pressure. That said, I have my front glad hands capped off; one of them was leaking a while back, so I removed them and capped them off (I don't anticipate it having to get towed I guess). I read your thread, but I don't understand what you did with them to get things reset. Can you dumb it down for me one more time? Thanks - Jason

 

[valkrior]

I found the answer to my issue, I had no working parking breaks, and the CTIS system wasn't working. Then I seen a spark from the electrical box. Once I Pulled it, there was one cable that was lose in the back. Once I fixed this, the parking brake worked as it should and so did the CTIS system. I spent countless hours trying to fix these issues, and it was one lose connection under the panel. It's about 10 minutes to get it off, take the time and see if everything is as it should be, that would be my recommendation.

Hey, just to be clear. You mean you had a loose wire under the fuse box on the passenger side dash (not on the driver's side)?

 

[Suprman]

Hey, I think I might be having the same problem you had. I jumped in today, and the brakes seem locked up. I don't think they are releasing. Plenty of air pressure. That said, I have my front glad hands capped off; one of them was leaking a while back, so I removed them and capped them off (I don't anticipate it having to get towed I guess). I read your thread, but I don't understand what you did with them to get things reset. Can you dumb it down for me one more time? Thanks - Jason

You cant block off either gladhand in front. If the blue cant breathe your brakes wont release. If the check valve going to the red one leaks you can release your parking brakes and the truck can roll away. You can eliminate the connection to spring brake release circuit on the red one and then cap it. But not the blue. There is a way to remove the two way check valve setup and then remove that gladhand.