M939 Front Brake adjustment?

[00t444e]

I am working on a 1982 M939 that the PO said the front brakes don't work that well and need adjusted. I pulled a front drum off and didn't see any adjustments, the drum and shoes are in great shape with plenty of material on them. Is there an adjustment for the front brakes or are they self adjusting?

 

[simp5782]

Self adjusting wedges. Ensure your wedge is kicking out like they are suppose to. They can rust and get gummed up and need lubrication to get them to move their full range of motion. You can remove the drum and apply the brakes and see what movement you have. May pull the wedge apart. Pull it apart. Clean it grease it and reinstall

 

[00t444e]

I pulled the drum off and had someone push the brakes and the drums didn't move.

 

[simp5782]

The shoes didn't move? Ensure you have air going to the chamber on the lines in. Are you having any air loss on the tanks with the brakes applied? You could have a blown brake diaphragm which will not allow them to apply. If you remove the line on the end of the can side and use an air gun or rig up a fitting and apply air the brake should engage unless the diaphragm is bad. If it's good then check the actuator rod in the chamber. If it's good remove the wedge assembly. They have a rebuild kit for them. Probably just needs lubrication though

 

[00t444e]

Yes the shoes didn't move not the drums, so which line do I apply air to? I have never worked with air brakes before.

 

[simp5782]

The one on the end cap. The tube that controls the wedge has 2 lines and a rubber diaphragm. the one on the frame side applies air. It forces the rubber diaphragm in and actuated the wedge and brake. The other line on the inside vents the air when released.

 

[topo]

I have never worked on military wedge brakes . I have worked on regular wedge brakes in trucks and I remember using a brake spoon or screw driver to adjust the shoes through the oval holes in the backing plates and turning the star wheel that moves the plunger out to move the shoe out closer to the drum

Some times the plunger could be stuck from mud and time or the star wheel could have the stars broken off from trying to be forced because its stuck .

The wedge brake works by the push rod from the air chamber pushes the wedge between the plungers forcing shoes a part and when they release they should self adjust by turning part the plunger assembly with the spirals on it that should turn the star wheels to make them self adjust . I'm thinking there is a small spring loaded locking tab to lock the spiral part .

 

[simp5782]

Rear brake setup. Front is the same it only pivots on one end though

 

[00t444e]

I rigged up a fitting so I could apply air to the actuator, and the wedge and shoes moved freely when I applied air. I believe that means I have an air supply issue.

 

[Jbulach]

I pulled the drum off and had someone push the brakes and the drums didn't move.

Make sure your helper wasn’t pussy footing the pedal, these trucks brakes are proportioned very heavy to the rear.

 

[simp5782]

I rigged up a fitting so I could apply air to the actuator, and the wedge and shoes moved freely when I applied air. I believe that means I have an air supply issue.

Or a bad q2 valve if its dumping air out the stack

 

[00t444e]

I put a pressure gauge on the supply line for the brakes and got no pressure at all when the pedal is pushed.

 

[simp5782]

Pull the middle line off of the q2 valve in front of the front axle and see if you have pressure at that supply line when it is depressed

 

[00t444e]

Pull the middle line off of the q2 valve in front of the front axle and see if you have pressure at that supply line when it is depressed

I pulled the line off and it has pressure going to it when the brake is pushed, but nothing is coming out to the wheels.

 

[simp5782]

Bad Q2 valve. O'Reillys # 229505CX or napa 229505x

 

[00t444e]

I replaced the valve and the front brakes work great now, thanks for the advice. Unfortunately shortly after pulling out of the garage it lost power and would hardly run, I open the hood and diesel was spraying out of the fuel shutoff valve, the arm had broken off. So any advice on where to get a new fuel cutoff valve would be appreciated.

 

[simp5782]

I replaced the valve and the front brakes work great now, thanks for the advice. Unfortunately shortly after pulling out of the garage it lost power and would hardly run, I open the hood and diesel was spraying out of the fuel shutoff valve, the arm had broken off. So any advice on where to get a new fuel cutoff valve would be appreciated.

Cummins # 208087

 

[74M35A2]

Make sure your helper wasn’t pussy footing the pedal, these trucks brakes are proportioned very heavy to the rear.

Whoa, you said the P word. Cool...