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Old 07-21-2021, 06:09 AM   #15
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Originally Posted by RealNiceTent View Post
There is a possibility I have not seen discussed and is starting to really make sense to me. If no air leaks can be heard and air pressure drops rapidly in operation as described the brakes could be way out of adjustment. The OP is describing a loss of pressure during slow maneuvering of the coach, and at stoplights. All of these conditions are low engine speed operation, meaning that air replenishment is at it's minimum. Also common to all of the conditions is a large amount of on-off input to the brake system. I know, wait a minute at a stop light the brakes are just on. This is true, but you are at an idle, and unless you are someone who drives air brakes a lot you will yo-yo the pedal because of the difference in pedal feel from a hydraulic system. On top of this there is creeping- the little start stop as cars move forward closing the gaps.



When the brakes are grossly out of adjustment each application consumes the limited amount of air a lot faster because the brake rod has to travel a lot further to apply the brakes to the drum/rotor.



On a coach the age of the OP's the probability the self adjusters in the brake ratchets have failed is very high, allowing the brakes to become grossly out of adjustment.
Great point, but I hope the OPs mechanic would have noticed the stroke of the brake chambers, while looking for any leaks.

Then again, maybe not !
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Old 07-21-2021, 06:20 AM   #16
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Have an air brake pro look at it. There are a lot of things that could be the issue, some more common than others. Let us know when you figure it out. Any part working normally won't be the problem, but one leaking or one that is misbehaving that you only notice when the brake is applied at low speed could well be your issue, no matter how directly related to pressing the brake pedal it happens to be.


Could be gremlins. Truth is we don't know without getting under your coach and looking for ourselves. Take our advice with a grain of salt.
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Old 07-21-2021, 10:39 PM   #17
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Real nice tent

What you are describing sounds very possible. We have not been able to find any leak. But, while parked at a standstill with parking brake on, I can apply the service brakes and watch both gauges drop fairly rapidly. And while at a stoplight, not really feathering the service brakes, that I'm aware of, and drops both tanks quickly again.
I've owned and driven the coach for many years, and just recently noticed this problem.
Would the wheels need to be pulled to check the adjustment? Perhaps excessive wear on the shoes?
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Old 07-21-2021, 11:29 PM   #18
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Great point, but I hope the OP's mechanic would have noticed the stroke of the brake chambers, while looking for any leaks.

Then again, maybe not !

We have no way of knowing, the OP didn't go into any details about his mechanic. He could be a qualified air brake mechanic, or he could just be the same guy that fixes his wife's car and the toad with a brakes are brakes attitude. Dizcom hit it on the head, OP needs to have an air brake pro look at it, if he is asking for help the probability that his mechanic is a certified or even qualified air brake mechanic are slim. My experience in the field has taught me that customers will frequently spend three times with me what the repair should have cost because I have to fix what their shadetree buddy "fixed". Sometimes you just have to bite the bullet and pony up for the pros to save yourself money and time.


And just so you know Conchajim, I'm not pinging on you, just stating my experiences. in forty plus years of working on everything from airplanes to Zamboni, including trailers I have had to learn just about every brake system there is: Air, electric, Hydraulic, mechanical manual, and hybrid systems. I still send certain systems to other people because I don't completely understand or know the systems well enough to repair them, like the electro-magnetic arrester systems that are used on some custom limos and limo buses to assist in reducing downgrade speeds.



In my opinion brakes are the most critical system in any vehicle. Quite literally brakes are Life or Death. If your mechanic is not regularly a heavy truck mechanic, you may want to have a full time heavy truck mechanic take over. A few hundred dollars to have a heavy truck mechanic look at it is cheap compared to the possibilities.


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Old 07-22-2021, 06:06 AM   #19
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What you are describing sounds very possible. We have not been able to find any leak. But, while parked at a standstill with parking brake on, I can apply the service brakes and watch both gauges drop fairly rapidly. And while at a stoplight, not really feathering the service brakes, that I'm aware of, and drops both tanks quickly again.

I've owned and driven the coach for many years, and just recently noticed this problem.

Would the wheels need to be pulled to check the adjustment? Perhaps excessive wear on the shoes?
If the air continues to drops off, with your foot steady on the brake pedal, its NOT an adjustment issue.

It can only be an air leak.
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Old 07-22-2021, 06:37 AM   #20
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Since an air leak is not heard, maybe the issue is with the air sending units that send the pressure readings to your dash.
Chock the wheels and air up the system, press and hold the brake pedal as you normally do when you notice the pressure drop. Release the parking brake, push in on the button. If you are indeed loosing pressure in the system, the parking brake button will pop out to automatically engage the brake at about 45-60 pounds. I forget the exact number right now.
I have not heard of this check mentioned in this discussion so far. If the brake button does not pop out, then you have a sending unit problem, in my opinion.
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Old 07-23-2021, 10:24 AM   #21
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Yes, we have done this. The parking brake will indeed pop out and set the brakes.
My mechanic is a certified air brake technician. He is a mobile big truck mechanic. Not so much a RV technician, but still qualified. He has just done a preliminary investigation so far. I'm just trying to get some possible answer.
The more I learn the better is how I see it. He very well might not not need my input, or want it!
That being said, I'm seeing this as a shsred system problem. Since both reservoirs are being drained. I'm thinking another possibility could be the brake light switch. It is supplied air from both systems.
Does anyone have any idea where it is located?
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Old 07-23-2021, 10:32 AM   #22
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Hard to believe that massive an air leak can't be heard.


Run engine until CUT-OUT PSI is reached. Turn off engine. In the silence, have someone apply the service brake while the tech listens under the coach. Doubt they will even need kiddie bubbles to find that size leak.
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