M1078 LMTV Parking Brake not engaging

[AzTinker]

I have a M1078 LMTV and the parking brake will not engage. When I pull it out the truck still rolls. When I push it in a small amount of air blows out. When the truck is shout down the brakes will lock after about 3 min. the time for the air to bleed off under truck. When I start truck up and air for front and rear get aired up per gauges the parking brake is still pulled out and tires roll free. I can push parking brake in and a small amount of air blows out. Just got truck from FT Bragg. Where to start? I have down loaded service books. Can my lines behind dash be hooked up wrong?

 

[sstephens637]

Mine is similar. Doesnt really roll free, but any kind of hill and it will start creeping.

 

[jmed88]

I am having the same problem. When I push it in only a small amount of air is released. I have checked all of my lines. Any ideas?

 

[tennmogger]

If air is dumping when the yellow valve knob is pulled out, sounds like the air system is doing it's job, specifically the spring brake relay valve. Maybe the three cases of poor parking brakes are simply needing the rear brakes adjusted? These are wedge-type brakes and there are good tutorials on youtube to show how they work and how to adjust them. I know I had a steep learning curve with air brakes.

I suggest the basic air brake tutorials because the manual is much harder to follow on how things are supposed to work (to me anyway).

May I compliment you on the title of your thread! It tells the type vehicle and the problem (perfect for searches in the future) and you wrote a good description of the fault.

 

[bikeman]

Are you sure the brake isn't caged? That'd be a quick check, otherwise start going through the brake adjustment portion. You should get a decent air hiss, but not too much... hard to describe without a video or similar...

Also a reason we use chock blocks with everything!

 

[Suprman]

If the brakes engage with the air drained but not with dash valve operation maybe the valve is bad. Does the pedal brake work properly?

 

[texfireman]

I have one releasing a small amount of air and breaks will not apply , Could it be a check valve ?

 

[Berend]

This is a common problem , I’ve sold a bunch LMTV’s here in Europe. I hat a couple with the same problem. If the parking brake doesn’t engage the check Valve behind the emergency front glad-hand is stuck and properly also leaking air. Problem water gets in and corrodes the inside. Remove the front skid plate to access it. Take it apart and clean it and it will
work again. I can take a picture tomorrow.

if your air bleeds out the of truck in 3 minutes you have a major leak somewhere. Properly the front emergency glad-hand also. I have trucks that hold air for more than 2 weeks .

 

[Ronmar]

Is the front emergency gladhand plugged or does it have a non-vented cover installed? The anti compounding valve controlls the park brakes. It in turn receives park control air from the valve in the dash via the inversion valve. The emer gladhand also applies air to the same point on the inversion valve via a 2 way checkvalve(thats how you release the park brake remotely from the towing vehicle).

If the emer gladhand in front is plugged/capped, park air leaking past that 2 way check or leaking past the one way check where the emer line adds air to the wet and service tanks, that emer line will pressurize and act like a storage tank. When you release park air with the dash valve to set the park brakes, the emer line will continue to feed air to/thru the inversion valve via that 2 way check and keep the brakes disengaged untill it leaks down enough to allow the springs to apply the brakes... I can post a schematic once I get home this evening...