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Old 05-10-2019, 05:18 PM   #1
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Unusual (to me) Brake Problem

My 05 Alpine 40 FDTS is in a trusted truck repair for a few items. I asked the owner to check the brakes as I felt that undue pressure was needed to stop. He found some glazing of the rotors and pads and took care of that. He noticed that the pedal worked fine, good braking, but if he held the pedal, it would gradually sink to the floor. Yep, he replaced the master cylinder. No change. He then said it was most likely a valve component in the ABS system. He pulled a number off it and found it's no longer available. He is calling in a friend that specializes in brakes to pick his mind on Monday. Any ideas what might be going on? Thanks in advance.
Mike
p.s. The coach has about 56k on it.
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Old 05-10-2019, 07:04 PM   #2
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I don't know 2005 coaches, and I don't have access to ACA documents to check, but I suspect you have the same brakes as I have on my 2001. The brake ststem is commen to school buses and commercial trucks. It is manufactured by Meritor Wabco and there is a ton of information available on the internet. I was a telephone man, not a brake guy, but I am mystified as to how an ABS component could cause the pedal issue you describe. My coach is currently sidelined because the ABS modulator valve has an open conductor to a solenoid. My guy was able to order the part. It is taking a couple of weeks because the factory is making the part. Here is a link to a repair manual for the brakes I have: https://www.google.com/url?sa=t&sour...Z4W4_KKjeT8CjN
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Old 05-10-2019, 08:03 PM   #3
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MM-0677 may be the right manual for you cosch.

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Old 05-10-2019, 09:38 PM   #4
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….agree with Andy, seems like a worst case, fluid would just by-pass ABS and go to calipers.….don't think there is a return line from ABS for a by-pass to go back to master....sounds like master could be leaking down.....or more likely there is air in the system....only sure way to bleed brakes is to remove all 4 wheels and draw new fluid thru all 8 calipers--starting with left rear....
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Old 05-10-2019, 10:03 PM   #5
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Would not the right rear be the farthest from the master cylinder?
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Old 05-11-2019, 06:10 AM   #6
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….I always get confused--is it the right side when you are standing in front or from behind? So how about passenger side rear?
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Old 05-11-2019, 09:48 AM   #7
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A new update on my android pad has been making it hard for me to share links. This morning I figured it out.


http://www.wabco-na.com/literature/Hydraulic-ABS
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Old 05-11-2019, 10:21 AM   #8
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Quote:
Originally Posted by Old Scout View Post
….I always get confused--is it the right side when you are standing in front or from behind? So how about passenger side rear?

LOL, that confusion is right up there with talking about the front or rear of the engine in a DP!


Typically, for this brake issue, you orient yourself in the direction of travel of the vehicle when describing which side something is on. In short, as if you are standing behind and looking at the vehicle.


Of course, when in doubt, call out the "passenger side." Let's not even get into RHD imports!
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Old 05-11-2019, 12:39 PM   #9
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I’ve always taken to mean from when your sitting in the drivers seat facing forward.
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Old 05-11-2019, 02:02 PM   #10
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We could use port and starboard, that would solve the second problem in this thread.

Back to the original, I’d agree with air in the lines. I recently had the same brake pedal sensation in a passenger car. It turned out to be a cracked booster hose. Since ours are hydraulic, you’d probably see evidence of a leak. Even so, we developed a crack in the hydraulic oil cooler last year, the one for brakes, power steering and cooling fan. We didn’t realize it was leaking until we stopped for fuel at the end of a 400 mile day up and over the Cascades. The toad was splattered so it had been going on for awhile. We drove it 4 miles home from the fuel stop and moved it a couple of times in the driveway. Never noticed a soft brake pedal feel. We did lose a gallon or so of the 4+ gallons in the reservoir.
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Old 05-11-2019, 02:51 PM   #11
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...degraded hydraulic pressure [a leak] would likely result in a harder pedal at some point...think I would focus on trapped air in line/calipers or M/C by-pass....moisture in fluid wouldn't show up until brakes were hot and water goes to vapor...probably good to check any and all bleed screws if they were loosened...
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Old 05-11-2019, 02:55 PM   #12
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Did he replace with a new master cylinder or rebuilt.

There have been more then a few bad rebuilds.
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Old 05-11-2019, 03:48 PM   #13
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1. replaced MC w/no charge?????? Wow.
2. all 4 calipers are approximately equidistant from the MC- the ABS modulator is about midships so pressure goes from the two ports on MC to the modulator then back out to all 4 corners.
3. I recall a case where an Alpine's brake pedal self depressed ~1/2" w/the engine running but returned to full up position w/engine OFF. Cause was hydraulic overpressure to the MC boost which was cured w/a new hydraulic pump.
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Old 05-11-2019, 05:11 PM   #14
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Thank you all for your input. I have forwarded to the mechanic.
Mike
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