Suddenly M1093 Brakes are stuck on

The truck was working fine. Then all of a suddenm, the brakes or parking brake are stuck on. The truck will go into gear and it tries to move, but seems held down. Every once in a while if I mess with the air brake engage knob, the brakes will get released and the truck can roll forward.

However, once I hit the brake pedal, its stuck again. PLEASE HELP!

Stuck valve in the system, IIRC. Check starting at the brake pedal and then all valves. I would also recommend a drain of the system (through normal means) and then refill it.
Reverse of this can happen in the FMTVs where the brakes won't engage due to water in the system that freezes in cold weather.

Thanks! When you say drain the system, you mean purge all the air out, correct? I don't think its air over hydraulic, so we are not talking about fluids.....

My 1078 just did the same thing, had to cage the rear brakes to move it. Going to start troubleshooting it, will be interested on what you find. Saw the truck the other day when I was picking up the heater / AC unit. Looked really clean. Very nice.

jpinst said:

Thanks! When you say drain the system, you mean purge all the air out, correct? I don't think its air over hydraulic, so we are not talking about fluids.....

Click to expand...

Yep, just open up the air tanks and let it sit, it'll be noisy, but take about 20-30 min for a full "exhaust" if the system is fully charged. After that, IIRC, actually pump the brakes a few times, open and close your e brake and trailer brake knobs. Make sure you have the truck FULLY chocked before doing this as you will loose all braking capability. between all of that it should empty the air out of the system. try recharging it then, and see if it solved the problem. AFter that you'll need to start trouble shooting valves and actuators via the TM.

Thanks...so far no luck.....I will have to use another truck for the event. I spent so much time on this darn thing lately. Oh well, Murphy strikes again.

Still suck...hating life....it must be a valve stuck on somewhere. Not sure how to tell.

do you have the full set of TM Manuals? they should walk you through step by step for a full system check.

I have the same problem on my m1088a1 rear axle, did you find the problem?

Could this be the wedge's sticking at the drum ? Just read something about that ?

One thing that helped me find a leak was to run the truck long enough to build full pressure, then shut the engine down and work the brakes and park valve. It's a lot easier to hear any leaks without the engine running. How much are the brakes holding the truck when they stick? There are two brake canisters per wheel on the rear, so if you had one sticking wedge that would only be 1/4 of the normal parking brake holding power...theoretically anyway. Then again a sticking wedge could also be on a front wheel if that were the problem. If you had the right flare fitting and cap, you could take the service line loose on one brake canister at a time and cap off the line, then try the brakes. You might be able to tell which one is causing the sticking issue. If more than one is sticking, then I would think it is something else like Bikeman suggested. Using the same methodology, you could take one parking brake line loose at a time and make sure all 4 rear canisters are getting pressure applied when you push in the parking brake valve. There are several checks like that you could perform, just a process of elimination. You could even jack up one corner of the truck at a time, apply the brakes, release them, and see if the wheel turns..might be able to at least see if the problem is just on one wheel. Just throwing ideas out there. I'm learning a lot about air brakes myself!

I fixed mine yesterday with a big hammer. The rear axle was locked up I was dragging the rear tires across the lot and it wouldn't break loose, tried forward and reverse, etc. last time I drove and parked it was a rainy day so at the advise of someone who does this daily and said the brake shoes rusted to the hubs, I beat the brake drums senseless with a big hammer and tap a bit on the shoes and sure enough the broke loose and the shoes popped free of the drums. Problem solved.

OK, I solved the problem a few months back, but I am just getting around to sharing the information, because I needed to find the graphics.

Here is the answer to stuck FMTV brakes....you ready?

Plugged gladhands or gladhands that are too tight (usually new). Bend them out a bit so some air can escape (to the point of a faint hiss when the truck is running) and problem solved....but I cannot tell you the H*E*L*L I went through trying to figure out the problem - not being a trained FMTV mechanic.

In fact, the Army even put out a cartoon FMTV locked brakes advisory noticed....take a look. This is who I figured it out...really. Anyway, thought I would share something useful since I am always asking for help.

Cheers all and thanks for all of your help int he past. Sharing info really makes these words work. View attachment Oct 2012 FMTV Locking Brakes_gtood.pdf

Thats why the front gladhands have the pressure relief valves on them. They look just like axle vents. Bugs make a nest in your gladhand and clog it up and your brakes wont release.