Air Brakes Question

Hi,
I have been driving my M929A2 more lately and got stuck in a traffic jam today. While sitting in traffic I was watching all the gauges and noticed something. If I left my foot on the brake pedal I would quickly lose primary air pressure. Is this correct? I realize that everytime the brakes are applied you use up air. But for some reason I thought the brakes would act like hydraulic brakes sitting in traffic. So, for example if I were stuck in traffic on a hill would I use the parking/spring brakes to stay put? Thanks in advance for any help on this topic and excuse my inexperience.

dodgemain said:

While sitting in traffic I was watching all the gauges and noticed something. If I left my foot on the brake pedal I would quickly lose primary air pressure. Is this correct?

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You have a leak somewhere. Not always, but the fist place to check is the diaphragm in the brake cans is first thing to check.

If you're loosing air pressure at a steady rate with your foot son the pedal, you probably have a leak at one of the brake chambers. Have someone step on the brakes for you while you check for leaks. I'm assumeing 929's have standard civilian type air brake systems.

You should not lose air pressure. there is definatly a leak somewhere. Could be a brake chamber or a foot valve. Build air, shut it off and step on the pedal. Have soldier B listen for the source of the leak. If it is a brake can, READ THE MANUAL. They can kill you.

WillWagner said:

If it is a brake can, READ THE MANUAL. They can kill you.

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You are right about that. I've had them apart and spring brakes have a massive spring in them that could very well kill you if you take the band off. You have to have a special tool to take them apart without injury. You can take non-spring brakes apart without the tool but they too have a spring in them. Unless you know what you are doing I wouldn't take any of them apart.

I would take no chances with this truck. It should be SOLD asap. I would be willing risk my own safety to help you dispose of this dangerous item.

Otherwise apply the brake pedal, find the chamber that is leaking, and change the rubber "pancake". They are sized by sq. inches, might be a #30. You can change a service side pancake by caging the brake chamber with the threaded rod that is in a holder on the chamber or get one where ever you get the pancake.
You can vise grip the hose to the service chamber to stop the leak long enough to get it back home if needed, been there, done that!
Also check the valves in back for the trailer, if it has them, may be open a bit.

WillWagner said:

You should not lose air pressure. there is definatly a leak somewhere. Could be a brake chamber or a foot valve. Build air, shut it off and step on the pedal. Have soldier B listen for the source of the leak. If it is a brake can, READ THE MANUAL. They can kill you.

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The brake pedal has o-rings inside the valve and will let air pass the o-ring when water or dirt get inside, you can check the leak by: build air, turn off truck, hold brake pedal down(if air gages drop, listen for air coming out the intake) if this is the case, remove the brake pedal and clean the inside.

Hi,
Thank for all the good information. Yesterday I took the truck for a spin around the storage lot to build pressure. Then turned off the truck and had my Dad press the brake pedal and I listened for leaks. I didn't hear any leaks and now the press isn't dropping. So, I think Sunday we will take for a longer ride and see if I can get the pressure to drop again. I will let you guys know what happens.
Thanks

HHHMMM ,If it is indeed a brake can (with park brake, sometimes called a piggy back can cause it is both service brake and park brake),sometimes those brake chambers can be sneaky little fellers---once in a while the rubber diaphram de-lams (leaks thru layers for better way to explain it) so sometime it leak sometime it dont.YES!!!AIR CANS CAN KILL YOU!!!Unless you are experianced or can take it to someone who is DONT TRY TO TAKE IT APART!! Just buy a new one -they aint all that spendy- easy to change as a unit.A lot less time than wandering around your neighbors back yard looking for your HEAD!You may be lucky--trhe leak could be in a hose going to the can.

Hey !!! Maybe we could get a new Emoticon thingy for these very critical safety items like the brake can issue on this thread.Maybe something like a skull and cross bones smiley face.Would be easier and quicker than the abbreviation I G F K Y. (It's Gonna F---in' Kill Ya)

waayfast said:

YES!!!AIR CANS CAN KILL YOU!!!Unless you are experianced or can take it to someone who is DONT TRY TO TAKE IT APART!! Just buy a new one -they aint all that spendy- easy to change .

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Eastern Marine has the diaphrams for next to nothing and the whole can assembly for less than $30 if it uses one of these types. Trailer Parts Superstore - Air Brake Chambers & Diaphrams

Kinda hard to go wrong with all new parts.

waayfast said:

Hey !!! Maybe we could get a new Emoticon thingy for these very critical safety items like the brake can issue on this thread.Maybe something like a skull and cross bones smiley face.Would be easier and quicker than the abbreviation I G F K Y. (It's Gonna F---in' Kill Ya)

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yea i agree , it may be time for such a emo ... good idea...and fun to boot...

Well, I put some miles on the truck over the past two weekends. The dropping primary air pressure problem has not returned yet. I wish it would just start leaking again so I could fix it.

On another note I did manage to truck the truck out of fuel. Man it was hard starting that 8.3 Cummins after that.

Maybe you could try wiggling air lines near the fittings while someone holds the brake, in case it is an intermittent leak in a fitting.

Also, the relay valves vent through tubing back to the intake stack, for fording purposes. The relay valve should only vent when you let off the brake, but I suppose it could be leaking intermittently while the brake is applied. You would hear the air releasing at the intake stack.

For running out of fuel, it is best to fill up your fuel filter with diesel first. It will still be hard to start, but at least you won't burn up a starter doing it. Do your fuel gauges work? I had one that was bad, and another was on its way (ck the ground first). But with new ones, they just beat themselves to death because there are no baffles in the tank. If they fail again on me, I'm not going to bother replacing them, just keep the tanks full as I do anyways.

i have baffles in the 819, had them in the m52 tank also