Air Brake system troubleshooting

Not exactly sure how to title this thread. I have done a ton of reading now in the TM's about the Air System and its parts and operation. I am still having a hard time wrapping my head around the whole thing and how all the components fit in. I am hoping someone with a working knowledge of these systems can give a general component description and what each individual part is supposed to do. For instance, the Pressure Protection valve was leaking and I tightened the upper nut down on the lower cap, but I'm not sure I should have done that because it is set at a specific pressure.

TM 2320-272-10

PRESSURE PROTECTION VALVE – Performs two functions:
• Allows air pressure to build to 60-65 psi (414-448 kPa) before supplying
air to auxiliary air-powered equipment.
• Closes off auxiliary air system from other systems if an accessory fails
and prevents loss of air from secondary reservoir.

Also the primary and secondary feed cutoff valves.

PRIMARY FEED CUTOFF – Manually-operated valve used to isolate
pressure leaks in primary air system from draining wet tank.

How would you use this valve to isolate pressure leaks.

Basically I have tracked down any leaks I can hear. I get pressure build up pretty quick to 120 lbs. I would say within 5 minutes. But when I depress the brake pedal pressure drops to about 90 lbs and my warning light goes off pretty quick. Also when I shut the truck down, pressure bleeds off pretty quick. Probably within a half hour I'm down to 20-30 lbs. I can't hear any leaks.

I am just trying to learn more about the whole system and how it works and would like some insight from those that have been through this. Thanks for any help.

Best thing for air leaks is soapy water. Be very careful with the brake chaimbers. They have one heck of a spring that will take your head off.

What model truck?

Sorry, M931 with ABS.

ABS, That means you have to have computer controlled servos somewhere. Out of my league.

Rick

This may not solve your entire issue but have you closed the glad hand valve(s)?

If your service valve is open, once you depress the brake it will let out a lot of air and quickly drop your tank pressure.

HI GUY 1 thing we found at the cal guard OMS shop was the brake valve over the rear axle's that feeds air to them (emerg and service) sides would develop a slow leak and also the brake chambers.
The best way to check is like sparky said water but the leak can also be on the service side,so try having someone hold there foot on the brake pedal and you can go under and see if hear any leaks.

steve

1 more thing, like D-Man said about the glad hand valves,the 939 series have them in the front too not just in back like the deuce's.

steve

It might be easier to learn a civi spring brake system 1st. Your truck has extra features in it, that civi trucks don't have.

The pressure protection valve, just gives priority to the essiential stuff 1st.

Do you have a tractor protection valve? Apply brakes about half way, block pedal w/stick in on position, and jump out, and check for leaks. 1st by ear, then with soapy water.

Do you have an air operated fan clutch? If so, that is a potential leak. The air line to it, should have a valve in it to shut it off, if not, put one in it! Figure out, how to make it a straight drive/no clutch (in case of clutch failure). Sometimes, you can just put a bolt in them.

The brake related issues, sound like a relay valve problem, and or a "quick release" valve, but probably the former. Even if you have electric controlled valves, due to the ABS stuff, you still have a similiar valve, to control the air.

The primary feed cutoff valve, sounds like a valve, or valves, to isolate and control a leaking system on a tactical MV, to keep it operating at all costs. A civi vehicle would just apply the spring brakes, and you would be parked untill you repaired the system.

If you build pressure to cutout and apply the brakes, and hold them on, does your system hold air pressure, or does it bleed off? If it does not hold, block the pedal on, and apply soap to all diaphram couplings and hose fittings.

Do you have a CDL, if not, it would be wise to have one. The hand books for the test, goes through the air brake system, and just reading it several times, give some good background.

Anyway, this will give you a little to go on. The books, always look different then the truck does. Its best to learn the stuff at home, otherwise, the truck is going to lock up somewhere in traffic, and you'll feel a little more rushed there. Good Luck

D-Man said:

This may not solve your entire issue but have you closed the glad hand valve(s)?

If your service valve is open, once you depress the brake it will let out a lot of air and quickly drop your tank pressure.

Click to expand...

Ditto


It may sound stupid but it is a commonly overlooked thing because it is so obvious.

Yeah thanks everyone. I have been doing a lot of reading also. Part of it is digging in to the TM. When I look at the operating system TM and the troubleshooting TM you get great information on how the system works, the function of each individual component and how everything flows together. I will dig in to it more this weekend with the troubleshooting info and hope I can figure out what's going on. Part of it is I was just looking at the repair and replacement section of the TM which does not tell you anything about how everything works. I had never looked at the operations TM or the troubleshooting TM. Now I have a whole new world of information to study and apply. Pretty excited about it.

I would download the P2P program: http://www.steelsoldiers.com/5-ton-up/82818-m939-p2p-program-ss.html

It has some really good diagrams that are animated.

Also, I would check the dump valve (I think that is what it is called). Mine tends to somewhat hang open at times. It will build pressure but it will loose it quickly when you apply the brakes often.

I have tried loading the P2P program several times with no luck. I'm not computer illiterate, but I just can't get it to load.

Dukeman said:

I have tried loading the P2P program several times with no luck. I'm not computer illiterate, but I just can't get it to load.

Click to expand...

No luck here either. I have Windows "7", any thoughts?

Thanks.

wheelspinner said:

No luck here either. I have Windows "7", any thoughts?

Thanks.

Click to expand...

I loaded it on my PC and I'm running 7. IIRC I had to stray just a bit from the install instructions but I think that I addressed what I did in the thread for it.

Dukeman said:

I have tried loading the P2P program several times with no luck. I'm not computer illiterate, but I just can't get it to load.

Click to expand...

Did you try the download from SoundGuy?

well I'm embarrassed to say, but I'm running Windows XP 5.1. Pretty lame hugh? I don't know if that has anything to do with it, but I haven't found the instructions very precise for my old version. I will go back and try again.

ok, got a chance to walk around the truck with a helper inside pushing on the brakes. Found the problem pretty quick. The shut off valve on the rear service glad hand was open, so every time the brakes were initiated the air was leaking out of the glad hand. Air holds fine now when brakes are on.

Question, there are no shut off valves on the front glad hands. Is there a check valve or something there that only allows the air to flow in from a tow vehicle?

I crawled around with a spray bottle and found absolutely no leaks at any connections, but i can hear a whining sound coming out of the Secondary tank. I don't see any leaks, but something is letting air escape the secondary tank. Could the check valve be bad allowing air to flow back to the wet tank?

Still losing air after shut down, only holds for about a half hour.

Air after shut down?

How long should air last after shut down?

Well longer then a half hour for sure, but if everything is working properly and all connections are sealed, I would think over night would not be too much to ask. I really don't have any experience with that, how long does your pressure hold?