M939A2 no air pressure - FIXED

[wehring]

OK folks this is showing how dumb I am but it is for the greater good of the group.

One of my M939A2 series trucks had an issue where it would not build air pressure. It is a great truck that I drive often and it has never done this before.

I ran out of diesel and decided to tow it because I was 7 miles from the shop. I pushed the override button on the dash to resease the brakes. I assisted at lights with my brakes while being towed. The truck was almost out of air by the time I got pulled around the shop and I left it for a few days.

When I got back in it I started it and it would not build air!!! The secondary would build to 30 psi but the primary would not come off of zero. I ran air through the gladhands and I heard it coming out of the intake.

Long story short... I got in it tonight to try to problem solve it. After repeating the same symptoms I held the brake pedal down while I ran the engine. It built air.

So... I ASSUME that when I used the last bit of air in the truck to stop at the shop there was no air left to reset the brake valve so it free flowed in the open position. I don't know this I am guesing. All I know is that it is back to being the same great truck it has always been.

Lesson learned. Hope this helps someone.

Justin Wehring
979 997 3112

 

[wreckerman893]

Ya got to hate that.

 

[jwaller]

this is what one of my trucks is doing, I'll have to try this method myself.

 

[Stonepicker1]

When I was a Tech at the OMS, I had to rebuild or replace the foot brake pedal a lot. Water in the brake system was the problem(not draining the air tanks)
The rubber o-rings inside the foot pedal would get gunked-up and let air pass through. Take it apart and clean/lube the o-rings.
The hard part is removing all the air lines on the backside of the pedal. After you do three or four of them its a piece of cake.

We also had problems with the check valves on all the air tanks sticking open(due to water in the system) The TM tells you how to do this check( we had to do this to all of our 5 tons do to a Safety message that came down from big Army ) Ended up replacing over half of them.

So make sure you drain your air tanks after operations