LMTV Parking Brake will Not Disengage

[Waltjg]

One week ago, truck drove fine, parked as always do; started up this week, released parking brake, and away we do not go. The brakes will not release! I followed the instructions in TM 9-2320-365-20-2 to troubleshoot the issue, but was not helpful. Air is reaching the wheels when the park-brake valved in the cab is actuated. Anyone have any suggestions? Thanks in advance...

 

[sandcobra164]

Hit the back drums with a hammer!!!

 

[Waltjg]

Yep, we think alike. Didn't help.

 

[Suprman]

Plugged up front gladhands

 

[Waltjg]

Thanks Suprman. I am aware this fault, and removed the gladhand dummies (covers) to test, no help. Does there need to be backpressure? I.e. the dummies need to be on with clear vents? Though the vents do seem clear--if I press on the vents with the air tanks full they let air out (not much though)...

 

[Suprman]

The gladhands don't function for air supply unless you activate the trailer control dash valve above the parking brake. There is a small check valv behind the fron grill its plumbed to the passenger side front gladhand. That's a popular culprit when brake release issues arise. I don't believe the gladhands need any back pressure. Try blowing air pressure in see if that does it.

 

[Waltjg]

I'll try that... Thanks again!

 

[TacticalTruck]

When mine did that it was the check valve under the left front of the cab.

 

[Waltjg]

Check vales behind the driver wheel-well are good; check valve behind the bumper is good; gladhands are not obstructed. As I mentioned in my original post, I am getting air at the spring port on the wheel chambers. This has got me all twisted up! Has anybody ever seen a schematic of the brakes system for the LMTV out there???

 

[Suprman]

The tm has troubleshooting steps but no full diagram that I have seen.

 

[Waltjg]

Been thru all of the troubleshooting from the manual--no help. Getting air at all the places it has you check--from the vale in the dash, all the way back to the chambers, and everywhere in between...

 

[Suprman]

Could you have a hole in a chamber diaphragm not allowing air to release the brake?

 

[NDT]

Try caging them to see if that releases the brakes.

 

[Waltjg]

I suppose it could be the chambers, I had just thought it unlikely because neither wheel is releasing. I should be able to hear the leak if I release the brakes, and get down there, right?

 

[Waltjg]

I'm ready to cage the brakes just so I can get it out of the shop! I was in the middle of a tranny drain and flush--I had drained, refilled and was trying to go on a drive to flush before I drained and filled again when this happened...

 

[Suprman]

Maybe your trans/tcase is having an issue not allowing the drivetrain to move. If you have air at your brake chambers maybe they are releasing.

 

[Waltjg]

Nope, it definitely the brakes. Trucks bears down on them when put into gear, and does its best to push against them.. Also, I can see that they are tight against the drum--all rear shoes. Thanks though.

 

[Suprman]

Just to clarify. When the truck takes away air from the spring brakes, it sends air to the service brakes (non spring like the fronts) which are in the same chamber housing. So just because you have air at the cans does not mean you have air at the spring brakes. I have heard of dash valves going bad.

 

[NDT]

Suprman, just to be clear: When you release the parking brake, air is sent to the parking brake chamber which compresses the spring and retracts the shoes. Later when you apply the service brakes, air is applied to the service chambers which actuates the shoes.

 

[Waltjg]

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!