Air Brake system troubleshooting

[juanprado]

Air Pressure

Not sure as I always idle it at least 5 min to warm her up but I think I loose the air pressure within hours after shut down. Never paid attention to it. I just thought it was normal. Was not sure how long it was supposed to last until I saw your post and was wandering if I have a problem or slow leak I was not aware of?

Can other M939 owners, chime in on how long you maintain air pressure after shut down?

 

[ARYankee]

My pressure will usually bleed off in a couple of hours. I really haven't timed it or paid too much attention to it. Usually if I'm parking it for the night, I drain the air. As long as there aren't any issues with it bleeding off badly while driving and not building pressure, I would worry too much about it.

 

[Greenbeast1]

Does anyone know what would be leaking in the rear two drums of passenger side of 923a2 when the brake pedal is depressed? I hear air leaking out from inside the drums when I press the pedal and then air gauge goes down pretty fast.
this truck has ABS

 

[212sparky]

Does anyone know what would be leaking in the rear two drums of passenger side of 923a2 when the brake pedal is depressed? I hear air leaking out from inside the drums when I press the pedal and then air gauge goes down pretty fast.
this truck has ABS

check the brake cans for leaks. it could be leaking air into the drums by the wedge

 

[Greenbeast1]

check the brake cans for leaks. it could be leaking air into the drums by the wedge

Thank you, I will check that out. Anyone out there in south fla that would be interested in working on a 923a2 in the service and repair business?

 

[Suprman]

Maybe the service brake diaphragm. You have to be very careful taking the spring chamber apart though. There should be a caging bolt on the chamber.

 

[Greenbeast1]

check the brake cans for leaks. it could be leaking air into the drums by the wedge

Where is the best place to buy the brake cans? The diaphragm seems to be leaking through the tube that is connected to the wedge

 

[212sparky]

I have seen cans on epay. Not sure if they are the same as yours. I picked up one for 90 shipped to go on an 86 international line truck 6 months ago. It looked the same s what the M932 have on the rears.

 

[Dukeman]

I am back on this after quite some time. We have tracked down and corrected several air leaks on the old girl but still have one giant problem. There is air leaking out of one of the rear brake diaphragms only when the brake pedal is depressed. The air system builds and holds air fine. It holds air for several hours after shut down. But if the pedal is depressed the rear can leaks pretty quick. How do I tell if it is the spring brake diaphragm or the service brake diaphragm? Can I change the service brake diaphragm without caging the spring brake? Can you tell me which TM would apply. I would like to read the TM but our internet speeds at work are so slow it takes a good hour to download each TM. If I know the correct one its no big deal, but to search through every one will take all day. Thanks for the help.

 

[wheelspinner]

Are you trying to read them online, or saving them to your computer? You need to save them down to a hard drive or CD to have any sanity at all left after trying to search. I don't know which one off hand, but if I get a few minutes I can check. Mean while, just start dowloading and saving them. You will be glad you did.

 

[quickfarms]

I am back on this after quite some time. We have tracked down and corrected several air leaks on the old girl but still have one giant problem. There is air leaking out of one of the rear brake diaphragms only when the brake pedal is depressed. The air system builds and holds air fine. It holds air for several hours after shut down. But if the pedal is depressed the rear can leaks pretty quick. How do I tell if it is the spring brake diaphragm or the service brake diaphragm? Can I change the service brake diaphragm without caging the spring brake? Can you tell me which TM would apply. I would like to read the TM but our internet speeds at work are so slow it takes a good hour to download each TM. If I know the correct one its no big deal, but to search through every one will take all day. Thanks for the help.

The spring brake diaphragm uses air pressure to release the brakes. If they are torn you will have to cage the brakes just to move the truck unless the brakes are out of adjustment.

You need to cage the spring brake to replace the service diaphragm. Even caged they are extremely dangerous because of all the energy. If you do not have experience working on the air brakes you really should have a professional work on it.

The good thing about the spring brakes is if you have a catastrophic air loss the brakes with automatically apply, provided they are correctly adjusted.

 

[quickfarms]

The COLA test is used to check the air compressor and air lines. This is a required part of your daily pre trip inspection. This is part if the CDL requirements.

Even if a CDL is not currently required in your state you should perform the COLA and pre trip inspection before the vehicle is driven each day.

On a practical note if the air leaks down enough over two hours to trip the low air buzzer then the leak needs to be found and corrected.

 

[RustyM923]

The service brakes are the smaller pots on the rear axle. They look like the fronts, just a little bigger. DP16 is the diaphragm size.


The spring brakes are the bigger pots on the rear axle. They have the caging bolt assembly on them. I would get a professional to help you if these are leaking air. They would be leaking air constantly if they were bad.

 

[quickfarms]

This is a civilian air brake manual but is is very informative just remember this truck has wedge brakes.

http://faculty.mccneb.edu/dlpartner/Documents/Air Brake Manual.pdf

I have posted this here several times.

It would be nice to add this to the TM section

 

[Dukeman]

Thanks!

 

[gstirling]

Question, while replacing my pancakes(diaphragms) on my front brakes (M936) there is a plastic guide that goes on the push rod (part 7 in TM 9-2330-272-34P-1 fig 184). the manuals do not show where it goes on the rod. does it go up near the piston or down near the wheel hub. mine were mangled up, also do they need any lube? or is the plastic good enough. I was thinking they help support the piston end and keep it centered in the rubber. or do they keep the rod centered in the wedge assembly at the other end? any advice would be appreciated. all the threads I could find focused on the diaphragm end.