FMTV Brakes are locked up?

[dsanderlmtv]

I did a thing tonight and I’m not sure if what I did was bad or did something mess up. After working on my M1078 A0 changing the passenger mirror, I released the parking brake and let the truck roll back against a wheel chalk about ten feet away, my driveway is relatively flat but I didn’t want the truck to roll over the wheel chalk so I applied the brake pedal. Let me add that the engine was NOT running but the truck had plenty of air in the system. Here is where things went wrong, my truck stopped just before the chalk so I started it up and put in reverse but it wouldn’t move under idle so then I put it in drive and the truck was locked up, even when I gave it throttle it struggles to move. I jacked up the front end and the tires are locked up, we had to leave to go eat dinner and a couple hours later the tires would spin free. I started the truck and let air build to full pressure and the tires still would spin UNTIL I applied the brake pedal, now the brakes are locked again. What in the world did I do or what went wrong?
Let me add that I think ALL four corners are locked up not just the front but I don’t have enough jack stands to get the entire rig off the ground.

 

[wheelspinner]

Start with your front glad hands. Them not venting is exactly what does this. Could be other things-but start simple.

 

[dsanderlmtv]

Start with your front glad hands. Them not venting is exactly what does this. Could be other things-but start simple.

Oh man…this makes perfect sense! Two days ago I capped off the front glad hands, just because I never dreamed of needing them. I will definitely try this first, thank you.

 

[Mullaney]

Oh man…this makes perfect sense! Two days ago I capped off the front glad hands, just because I never dreamed of needing them. I will definitely try this first, thank you.

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Yep, put it back like it was.

Those glad hand covers have vents on them.
Even flying bugs that make nests with mud will stop up the vents on the front of the truck.

 

[dsanderlmtv]

.
Yep, put it back like it was.

Those glad hand covers have vents on them.
Even flying bugs that make nests with mud will stop up the vents on the front of the truck.

Thanks for the help, the front glad hands are back on and everything is working like it should.

 

[Ronmar]

And since it locked up so readily, you probably have a bad 2way check valve. with full air, have someone step on the brake pedal while you observe how much air is leaking out the front blue service glad-hand. That lost air is not going toward braking and is depleting your braking air supply making the compressor work more to replace it.

Those 2way checks are located Under the drivers floor/behind the drivers step. Their sole purpose is to interface the glad hands to the brake systems. The top one is for the park brake and interfaces the red emer glad-hand into the park brake circuit. The bottom 2 are tied together and interface that front blue service glad-hand into the primary and secondary service brake circuits…. They are very easy to bypass safely if you are not using the front gladhands. The service brakes take about 5 minutes and one wrench and is completely reversible without any modifications. The park brake takes a few minutes longer with the grill removed, but also no permanent modifications required…

 

[dsanderlmtv]

And since it locked up so readily, you probably have a bad 2way check valve. with full air, have someone step on the brake pedal while you observe how much air is leaking out the front blue service glad-hand. That lost air is not going toward braking and is depleting your braking air supply making the compressor work more to replace it.

Those 2way checks are located Under the drivers floor/behind the drivers step. Their sole purpose is to interface the glad hands to the brake systems. The top one is for the park brake and interfaces the red emer glad-hand into the park brake circuit. The bottom 2 are tied together and interface that front blue service glad-hand into the primary and secondary service brake circuits…. They are very easy to bypass safely if you are not using the front gladhands. The service brakes take about 5 minutes and one wrench and is completely reversible without any modifications. The park brake takes a few minutes longer with the grill removed, but also no permanent modifications required…

Thanks for the response, I’m definitely going to look into this. I will keep you posted on my results.