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Old 10-17-2013, 06:45 PM   #1
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Pressing brake pedal causes continuous drop in front brake pressure - Normal?

I had a look at a 2000 American Coach with Spartan chassis today. When I held down the brake pedal, I could hear hissing from the front end and the front air pressure continued to drop for as long as the pedal was held down. The dealer assured me this was normal, but I don't remember seeing this happen on other DP's. the rear pressure did not continue to drop. Obviously, I have little experience with air brakes and would appreciate your input.

How long should the front and rear tanks hold their pressure after the engine is turned off?

If the tanks are empty, how long should it take the compressor to fill them? Less than several minutes?

Most likely places to search for a leak? How expensive a problem could it be (best SWAG)?

Thanks.
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Old 10-17-2013, 07:24 PM   #2
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It probably has a blown diaphragm in one of the front brake cans.......it should not hiss or lose pressure with the brakes applied........ JB
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Old 10-17-2013, 07:29 PM   #3
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The f/r systems should hold air for days with the engine off.......mine falls to 65 after a few days (the suspension bleeds it off) but I have gone back after weeks in storage and it's still @ 65. Shouldn't take more than a couple minutes at idle to reach the cut-out pressure and dump the air dryer........... JB
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Old 10-17-2013, 08:36 PM   #4
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While pressing down on the brake pedal you should not lose any air pressure, only when releasing the pedal does it release air. I would not believe the salesman and have it checked out for sure.
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Old 10-17-2013, 08:44 PM   #5
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I agree with pivotdoc, there is an air leak that must be repaired. This Utube video shows exactly how to perform an air brake test for proper operation.CDL Pre-Trip Air Brake System Live Inspection.wmv - YouTube If air pressure drops more than 4 psi per minute it will not pass a DOT inspection. The only difference in the video is, a MH system does not have a trailer brake set button.
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Old 10-17-2013, 09:00 PM   #6
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Quote:
Originally Posted by pivotdoc View Post
It probably has a blown diaphragm in one of the front brake cans.......it should not hiss or lose pressure with the brakes applied........ JB
X2 on that.
I'd find new dealer as well.
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Old 10-17-2013, 10:23 PM   #7
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To answer part of one of the original questions, rebuilding or replacing a brake can is not that expensive, especially if you do it yourself. I agree with the thought of a different dealer though, if he's willing to lie or bluff that it was perfectly normal, what else is he lying about?
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Old 10-18-2013, 10:29 AM   #8
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Quote:
Originally Posted by pivotdoc View Post
It probably has a blown diaphragm in one of the front brake cans.......it should not hiss or lose pressure with the brakes applied........ JB

I would include a few more suspects, but what he said.
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Old 10-18-2013, 11:32 AM   #9
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I forgot to mention, a friend had a rear brake lock up, blown can. It was replaced on-site by a mobile repair service. Cost for a new can, $35 ea.; labor was 2 hr minimum and repair was completed in < 30 min.
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Old 10-18-2013, 11:32 AM   #10
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Thank you all for your help. I believed this was a problem, I just needed to hear it from more experienced voices. Since it was the service manager that said this was normal, I agree that we are dealing with credibility issues. But then, do I want to go RVing or searching for the perfect dealership?

Can someone offer a ballpark on parts and labor to replace a brake can?

The other issue is that the ABS light is on. I did not mention it earlier because I have read that the light may go out once the coach moves, but that was not the case. This could be caused by something major or minor. If you have experience with ABS problems, how much 'insurance' would you build into the offer? Yes, I could ask them to fix it, but based on the discussion so far I would prefer to have it done right myself.

Could the ABS light and leaking front brake be related?
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Old 10-18-2013, 03:45 PM   #11
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My ABS light stays on. The ABS system does not function when the light is on, the brakes still work. Can't speak about and air brake system, but when DW's car ABS light comes on she simply runs her car through an automated car wash that also washes the under-carriage. That usually turns off the ABS light. I posted #9, ballpark costs for the air chamber at the same time you posted #10.
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Old 10-18-2013, 04:00 PM   #12
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I don't know if ABS error would light because of a bad air chamber. ABS problems are usually due to a sensor falling off bracket behind wheel or being torn away by road debris. ABS light usually means you've lost anti-lock brake function, brakes still work, just not as well as with ABS. I don't know the cost to repair, sensors run $20 on up, according to brand.
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Old 10-21-2013, 08:50 PM   #13
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The brake can has been replaced and waiting for parts for ABS issue. A third problem I have noticed is that the gauges never reach 125 psi. The front tops out around 118 psi and the rear around 110. I never hear the over-pressure valve kick out. This could imply additional leaks, or could it be that the compressor cannot produce 125 psi?

Any compressor experience out there? Thanks.
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Old 10-22-2013, 06:56 AM   #14
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When was the last time you changed (or had someone change) your air dryer desiccant filter?? If it's been over 2 or 3 years it probably needs changing. And Yes it will cause the Air System not to come up to normal pressure. Also, when did you last purge your Air Tanks? They should either have lanyards or automatic moisture ejectors.
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