M939A2 series dragging brake

charlesmann

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My l/h front brake was dragging/applied and after applying some air tool oil to the service chamber, with no luck of releasing the shoes, I removed the service chamber, and still had no luck getting the shoes to release. I was able to air hammer the drum off, and in the process, noticed the hub was loose, which is probably why the inner bearing/hub seal stayed on the spindle. I rmvd the hub, rmvd the shoes, disassembled the wedge system, cleaned, inspected visually, greased with red n tacky bearing grease and reinstalled. I cleaned and packed both bearings, since the inner bearing was a lil discolored, instead of the bright red the grease normally is. When installing the svc chamber, i heard and felt a thump in the tube of the chamber. When tightened the bearing nut for initial seating, i was able to sing the hub freely, of course till the true on the bearing nut high enough. I installed the hub/shaft cap and wheel, and as I was testing the operation of the brake, I could feel a grabbing in the drive train, then it would free up for about 1/3-1/2 a rotation of the wheel, rinse and repeat.

Any ideas what could be causing the grabbing and freeing of the drive train?

Also, I would assume the brakes are NOT self adjusting, correct? I may have thought i fixed the brake dragging, but on a 30 miles test drive, but i noticed the left front drum getting warmer, the longer the drive went on. I did stop about every 5 miles and check all 6 hubs for warmth, and by the time i got to TSC and back hm, I noticed the left front drum seemed to be slightly warmer than the other hubs, making me think the sticking/dragging of the shoes is not fixed. The last 2 turns, i used gearing and very, very minimal braking and no brakes pulling in to my drive, applying only enough brakes to come to a very quick stop.
Could my problem be in the air system itself? If so, why only the left front? I did noticed the rear axle, on both side were warmer than intermediate axle hubs, which i may have a similar/same issue, just not as severe as the L/F, but enough for me to tear it down and investigate. I did jack both sides of the front axle up after a short 1 mile test drive, and felt no play in the hub, did the same for the rear axle, with same results, but thinking the weight of the wheel assembly might be preventing me from getting a good check, but i did use a pry bar to pry up on the tires, while pushing in on the top, and got no movement.

Thoughts, theories, suggestions?
 

simp5782

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They are all auto adjusting wedges.

Remove the lines at each front and plug the sending line and see if it grabs. If it does cap off both front hoses and see if it does.

If it doesn't then you have a Q2 valve or a treadle valve sticking. Treadle valves are known to slowly apply brakes while driving as well as being slow to vent their pressure.
 

charlesmann

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I think i will try whacking the treadle valve with a hammer, or simply just replace it and give that a try.
What is a Q2 valve, location?
 

simp5782

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I think i will try whacking the treadle valve with a hammer, or simply just replace it and give that a try.
What is a Q2 valve, location?
Front of the truck, below radiator on the bracket there. They usually go bad and vent to the stack with the pedal depressed but they are the activating equalizer valve for the front axle brakes.

You can put a air pressure gauge rigged to the air lines to the chamber and see how long the pressure holds after the pedal is released.
 

charlesmann

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Front of the truck, below radiator on the bracket there. They usually go bad and vent to the stack with the pedal depressed but they are the activating equalizer valve for the front axle brakes.

You can put a air pressure gauge rigged to the air lines to the chamber and see how long the pressure holds after the pedal is released.
I will take a look and see how long it holds pressure.
 

charlesmann

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@simp5782
I wasnt able to tee in a gauge to my L/F svc brake bc i forgot to get a close nipple. But i had my wife apply the brakes with the discharge hose disconnected at the chamber and frame.
when brakes are applied, the discharge at the chamber and frame puff a lil air and when brakes released, the chamber creates a suction and the frame port does nothing.

Normal or something jacked up in the system?
 

charlesmann

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No psi registering or holding on a 0-150 psi gauge on the exhaust side after release of brake.

Up to 30psi on inlet side during application of brake.

Tank psi was 120 pri and 125ish sec, with truck running.
 
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