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M923 air pressure problem

simp5782

simp5782

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Yea especially all the sudden. I am not suspecting them at all to be bad. . Just took this video. No brakes cages. E brake off. Doesn't take long to build pressure. But while driving and I come to a stop I tend to always have a brake squeal from the rear. I do not have it now. FFW to about 1 min into the video.

https://youtu.be/yXUjXRCpKQQ
 
MtnSnow

MtnSnow

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Replace the Q-2 valve that is in front of the front axle mounted on the plate that has brake airlines going to it. I bet if you take it apart the plastic sealing ball has broken apart.
 
simp5782

simp5782

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Will do. I will add that my abs light as always been on. And brakes would lock up if applied hard. Force the pedal to the floor and they wont lock up at all. Light is still on.

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Last edited:
MuleMac01

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Most of the time if you hear air leaking into the air intake.. check the break pedal had this happen to a 931A2 something failed inside the pedal causing it to always lose air, And it vents into the air intake.
 
simp5782

simp5782

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Ok after Q2 valve was installed it was a very quick drain of both tanks. Installed a good used treadle assembly and i still have the same problem. Not as fast of an air loss with the old treadle assembly but still will not hold at with brakes applied and still hissing out the intake.

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simp5782

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Alright well so far today I replaced the Q2 valve on the front. This now allowed air to be drawn from both tanks when I hit the pedal and it vented out thru the intake. After doing this the air pressure dropped very quickly from both tanks 120psi to 0 in a 10 seconds.

Next I Replaced the treadle valve. Same result. Hear air venting thru the intake. air loss is slower than before but still drops from both tanks with the pedal depressed. Letting the truck roll on a slight incline, pedal pushed and will not stop truck.

Next I had a good take off step panel that had the 2way valves # 3 and # 4, Quick Release Valve and a Inversion valve all connected. So I installed it onto the truck. Still has the same result.

Shut the primary take valve, brought the secondary system up. Hit the pedal, still vents into the intake and vise versa with the secondary shut off valve.

I still have brake lights so I know air is getting to that check valve. I will check the output of that tomorrow. System is holding air pressure after shutdown for 30+ mins.


Does anyone know what air line # lets it go back into the intake? I am using the P2p program and do not see it.
Also when hold the brakes and the driveline brake is applied the truck will roll forward on its own. The driveline brake will hold the truck on a hill and is adjusted properly
 
Last edited:
Suprman

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The abs has an inversion valve. Not sure if that would cause your symptoms though. If you had holes in your service brake diaphragms when you stepped on the brake it would cause a fast air loss.
 
simp5782

simp5782

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WYomer said:
so on the video whats up with your temp gauge being pegged?
Click to expand...
It is shorting out somewhere between there and the temp unit. It usually tends to sit right at the 240 mark even though the engine is only at 180. I just haven't replaced the wire, it could be as simple as the unit not grounding as well.Also strange that If I swap both wires off the gauge onto the brand new trans temp gauge it doesn't register anything at all. What is really strange is that my temperature gauge wouldn't read at all with a new gauge and a new sender until after I replaced my shifter cable and had to rewire some things on that side bracket of the transmission. Yet none of those wires have anything to do with it.
 
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WYomer

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simp5782 said:
It is shorting out somewhere between there and the temp unit. It usually tends to sit right at the 240 mark even though the engine is only at 180. I just haven't replaced the wire, it could be as simple as the unit not grounding as well.Also strange that If I swap both wires off the gauge onto the brand new trans temp gauge it doesn't register anything at all. What is really strange is that my temperature gauge wouldn't read at all with a new gauge and a new sender until after I replaced my shifter cable and had to rewire some things on that side bracket of the transmission. Yet none of those wires have anything to do with it.
Click to expand...
Maybe a common ground or you just bumped something. Good luck tracing all that.
 
simp5782

simp5782

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WYomer said:
Maybe a common ground or you just bumped something. Good luck tracing all that.
Click to expand...
Power comes in to the gauge and goes out of the gauge to the sending unit. Sending unit is the ground and the resistor in it makes the temp read on the gauge. More than likely the unit just isn't grounding due to rust or some crap in the threads.
 
simp5782

simp5782

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Ok, So for this afternoon I replaced the Quick Release valve since it was just easy and quick to see if it worked. No dice. So I went ahead and replaced all 10 service diaphragms and also the 4 spring diaphragms since I had em off anyways. None of them were damaged other than some slight cracking and wear. After the truck aired up and the brakes applied the primary tank no longer loses air but holds steady, the secondary tank however still vents off thru the intake. So apparently I have a few problems. The secondary tank never would pull over to the brakes before I replaced the Q2 valve so apparently a good many brake problems at once. Will work on the secondary tomorrow About to be a few busy days of transporting with heavy loads so I need it all up and going.
 
PureMayhem

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I have the same problem it seems. I have air exhausting through the stack and it is coming from thru treadle valve. I have narrowed it down to a problem in the secondary air system using the troubleshooting steps listed in TM 9-2320-272-23-1 for the ABS system. That is as far as I have gotten so far.

simp5782 said:
Alright well so far today I replaced the Q2 valve on the front. This now allowed air to be drawn from both tanks when I hit the pedal and it vented out thru the intake. After doing this the air pressure dropped very quickly from both tanks 120psi to 0 in a 10 seconds.

Next I Replaced the treadle valve. Same result. Hear air venting thru the intake. air loss is slower than before but still drops from both tanks with the pedal depressed. Letting the truck roll on a slight incline, pedal pushed and will not stop truck.

Next I had a good take off step panel that had the 2way valves # 3 and # 4, Quick Release Valve and a Inversion valve all connected. So I installed it onto the truck. Still has the same result.

Shut the primary take valve, brought the secondary system up. Hit the pedal, still vents into the intake and vise versa with the secondary shut off valve.

I still have brake lights so I know air is getting to that check valve. I will check the output of that tomorrow. System is holding air pressure after shutdown for 30+ mins.


Does anyone know what air line # lets it go back into the intake? I am using the P2p program and do not see it.
Also when hold the brakes and the driveline brake is applied the truck will roll forward on its own. The driveline brake will hold the truck on a hill and is adjusted properly
Click to expand...
 
simp5782

simp5782

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PureMayhem said:
I have the same problem it seems. I have air exhausting through the stack and it is coming from thru treadle valve. I have narrowed it down to a problem in the secondary air system using the troubleshooting steps listed in TM 9-2320-272-23-1 for the ABS system. That is as far as I have gotten so far.
Click to expand...
Take the front Q2 valve apart and see if the nylon ball is busted or if the spring is bad.

https://youtu.be/GG9H5Zc52qM

If you have replaced it already then swap your drivers side middle and lower line with each other.

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