m51a2 Brakes will not release after driving a distance

[mbware]

I have a 5 ton dump truck where all the brake componets have been replaced. Master cylinder, hydravac, wheel cylinders etc. Brake master has more than required 1/2 inch slack. two new hydravacs have been installed. Around the field the brakes work perfectly. stops on a dime and releases pressure properly. Put the vehicle on the road and travel variable distance of 7 to 20 miles at speeds of 45 mph or greater and the truck stops fine but retains air pressure on the brake light switch and the booster... thus the brakes are locked. Bleed master cylinder at bleed nipple without any release. Pressure must be released from outlet on booster to release. Anyone have a clue as to the problem? Compressor, pressure regulator and all else check out fine. Thanks for any clues

 

[Westech]

the vent tube? I just did a down pipe on mine and not vent it in to the system

 

[doghead]

Welcome Mbware, you need to add your location to your info/ profile (site requirement).

 

[mbware]

Has a vent tube that turns down from top of Booster. No fording kit on truck. Breaths great and sounds just like my 2 other 5 tons

 

[DaveP]

first off be sure to add your location to your sig line. on the brake issue, my XM818 had the same issue when i bought it.matter of fact the prev owners made sure they showed me where they left the wrench to bleed of a little fluid when it locked up. turned out to be the glad hand cover on the right front had the little vent hole plugged and was retaining pressure. i know it does,nt make a lot of sense but after cleaning the vent it works just fine for 3 years now.I admit that it drove me nuts for a couple days until i figured it out.

 

[jatonka]

check the vent on the glad hand cover on right front fender. if plugged, your brakes will lock when applied. Had same problem on my M51A2, replaced everything with new parts, had to bleed every time I used the brakes. Pulled some hair out, listened to Wrecker One from NJ and fixed my problem. JT

 

[acetomatoco]

That special glad hand cover for the right front comes in two versions...one has a differential type vent on it and the later one has a little hole in the center... both performing the same function... letting out air which creeps by the check valve for remote application of brakes when towing. That line needs to breathe.

 

[mbware]

Thanks a bundle for the responses. Truck was missing glad hands on the front when I bought truck and replace them with general commercial units. Will try the suggestions and find vented unit. MANY Thanks

 

[m139h2otruck]

This is somewhat the same problem we have with the water truck, but the brakes do not lock or bind. Tried venting the right front line (no glad hands) but the brake lights still stay on, and no air comes out. Takes between 1 and 5 minutes to go out, depending on how hard the brakes are used. MC, brakes and hydroboost all in good shape.

 

[jatonka]

mbware, you don't need to buy a special part, just drill a small hole in center of the one you have on it. Start with a 1/16" hole. You'll be all set. JT

 

[mbware]

I have 3 of these trucks running and a lot of parts from a couple that have been parted. Already put a glad hand on it with the capped vent.. just haven't driven for test ride yet. I'm sure it will work now especially when you figure how tight the seal was on the system with the all new glad hands components.
Thanks again,
Mike

 

[acetomatoco]

Deuces do not have the special set up allowing remote application of brakes, so the info just relates to 5 ton and larger...don't know about the A3, too new for my brain bank. For the slow release on the deuce, I would expect collapsed rubber hoses, or a lazy airpak... be sure to put the OHT in the air pak twice a year...keeps the rust from the wet air from binding it up...