All four rear brakes sometimes leak even when brakes not pressed.

[dannyd2450]

Ok, if the parking brake is on, pressure builds and maintains at 120 psi. When the parking brake is released and the truck is placed in neutral, all four rear brakes sometime leak. This happens when the parking brake is released and the brakes are not pressed. It doesn't happen all the time, just sometimes and as you can imagine, 120 is hard to achieve with 4 leaks!

Each can leaks exactly the same amount. I pulled the rubber nipple off and the hissing was exact on each unit.

Could it it be the abs system of the rear axels and brakes? If I read right, there is an abs unit that controls the rear brakes: one unit for the four brakes? Could that have a valve that gets stuck?

or is it one of the valves somewhere else that is staying open?

 

[porkysplace]

Ok, if the parking brake is on, pressure builds and maintains at 120 psi. When the parking brake is released and the truck is placed in neutral, all four rear brakes sometime leak. This happens when the parking brake is released and the brakes are not pressed. It doesn't happen all the time, just sometimes and as you can imagine, 120 is hard to achieve with 4 leaks!

Each can leaks exactly the same amount. I pulled the rubber nipple off and the hissing was exact on each unit.

Could it it be the abs system of the rear axels and brakes? If I read right, there is an abs unit that controls the rear brakes: one unit for the four brakes? Could that have a valve that gets stuck?

or is it one of the valves somewhere else that is staying open?

It might help to post what model truck your working on.

 

[gimpyrobb]

Pancake diaphragms.

 

[rosco]

The only pressure there would be to the Spring Brake side & they should not leak at all. Are they permanently sealed, or do they have removable clamps? In any case, there are real mean springs inside those cans, and you need to know what your doing, before messing with them. A picture and some truck ID would help.

 

[dannyd2450]

Sorry, It's a 89 M923.. That's what baffles me. When the parking brake is off and the brakes are not applied is when it leaks (sometimes)... As for springs, I have heard the horror stories, so I appreciate the heads up. The only thing I did was pull that rubber tab off to verify the interior was leaking (which the four rears do, sometimes)...

 

[dannyd2450]

Anyone have any experience with the treadle valve? As I understand it, this valve is the primary point from the air tanks to the brakes. Bare with me... If its the problem and I disconnect the line on the output side, I should be able to hear air coming out even if the brakes are not applied?

If no air is coming out until i press the brake, it works fine.

The valve operates at the pressure applied; more you push on the brake, the more psi builds. So, I should be able to just slightly tap the brake and not get a rush of air come out.

I also will tap into my compressor so I dont have to turn the truck on and can listen better.

thoughts?

 

[dannyd2450]

Bump

 

[davo727]

Right. When you release the parking brakes it ports air to the spring brake cans which compresses the springs and disengages the brakes, so if the diaphrams are bad then that's when they are going to leak. ( no air goes to them when the parking brakes are on , they are vented to ambient and the springs apply the brakes )

Sorry, It's a 89 M923.. That's what baffles me. When the parking brake is off and the brakes are not applied is when it leaks (sometimes)... As for springs, I have heard the horror stories, so I appreciate the heads up. The only thing I did was pull that rubber tab off to verify the interior was leaking (which the four rears do, sometimes)...

 

[Tinydragon2001]

Yep number 16 diaphragm. http://www.steelsoldiers.com/showthread.php?117662-Replacing-Spring-Brake-Diaphragms

 

[Artisan]

I have a gladhand w/ a male QD nipple I connect to my shop air line
to fill my brakes at the DS front Gladhand. (Open valve if present on truck )

So CHOCK WHEELS real good first on flat ground and filler up w/ air and
have solider B work the treadle and E-Brake etc while you are under
listening for leaks. Like gimpy said, pancake diaphragms I bet.

Those cans are only like $50 EA, the pancakes are like $6 I thunk
but BE CAREFUL, be POSATIVE you are doing it right, even make
sure your cage bolt's ends are 100%.

It won't hurt to open the TM's and study up too.

 

[1 Patriot-of-many]

I find it odd that all four diaphragms are leaking think you need to isolate the sound better