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Old 05-06-2015, 09:57 AM   #1
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Mysterious brake problem, any suggestions?

I'm hoping someone has seen this before and can point us in the right direction. 1994 Winnebago Brave 29RQ Chevy 454 on a P30 chassis.

2012 our brake light went on in Nov. when we went to take it for the monthly drive. We took it to the shop and they said the master cylinder was leaking and replaced it. All was good through the summer.

Following year the same thing happened shortly into the storage time during the winter. We took it back to the same folks. They replaced the master cylinder on their dime.

This past winter same thing happened. We refilled the brake fluid and continued to test drive. Noting that it went down a little each test drive. Tried to observe a leak. Found none.

Took it to a different shop this last week. They also could find no leak. They thought maybe the cover was loose and fluid was sloshing out. We are having the lines bleed and they are looking into whether the cover can be replaced.

Does anyone have any experience with a similar problem or know of any thing else we should have them check/do while it is in the shop? Being the brakes I would like to have a definitive problem identified with fix rather than just shotgunning it and hoping we fix the problem.

Thanks!
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Old 05-06-2015, 11:16 AM   #2
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I would have a serious look at the seal between the Master Cylinder and the Brake Booster. Sometimes fluid can leak from the MC into the booster. The vacumn connection from the booster to the intake manifold sucks the fluid into the engine and burns it off.
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Old 05-06-2015, 04:04 PM   #3
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Thanks! Had them check this. They said the two units were integrated so they are going to replace the master cylinder again. Crossing my fingers that is it!
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Old 05-06-2015, 07:40 PM   #4
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I also suspect the booster rather than the master cylinder. Whenever I have not been able to find the leak it almost always ends up that the booster is full of fluid. Highly unlikely that you had 3 master cylinders leaking with no evidence of the leak.
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Old 05-06-2015, 09:23 PM   #5
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The older P30 chassis were known for bad rubber brake lines. look at all the rubber lines to make sure they are not seeping. I had a 97 that had this problem. lines had to be custom made.
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Old 05-06-2015, 09:56 PM   #6
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While this may seem to be a problem there are some things that are obvious about this situation. I'm agreeing with poster #4

Brakes build very high pressures, well into the 1,000 PSI range and better. It is a sealed system. If fluid is leaking and there are no external signs of fluid dripping or wetting hoses etc, then it has to be leaking into the vacuum booster and sucked into the engine.

After over 40 years working on cars/trucks I have never seen or serviced a proportional valve until last week. A piece of a rubber hose internally broke off and plugged the valve. I have also never seen or heard of a vacuum booster sucking fluid from the brake system. Common sense tells me if it's not leaking externally and your fluid level is dropping it can only be going into one location. I don't care what anybody thinks where else would it be going???? This is such an uncommon happening that most techs have never seen it and therefore don't want to go there or don't even think to go there.

If I'm wrong then I'll admit it. I can't think of any other place the fluid would just disappear unless it's going into the vacuum booster or into the engine.

Good luck on this one!! Do post what you find please.

TeJay
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Old 05-06-2015, 10:31 PM   #7
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Thanks all!

Just to get some more info so we can provide feedback to the mechanic. Ours is saying the brake booster is incorporated into the master cylinder assembly. Sounds like you are all saying it is separate and they should be checking the seal between them. Am I interpreting this correctly?

Also to clarify TeJay's post. Should we be inquiring about the proportional valve also? Or are you thinking also the seal between booster and master cylinder.

Were looking up all these parts as your posting about them so pardon our ignorance.
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Old 05-06-2015, 10:37 PM   #8
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Do you have rear disc or drum brakes? If drums one of the wheel cylinders may be leaking.
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Old 05-07-2015, 02:08 AM   #9
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Have them pull off the vacuum hose, from the booster and feel or smell for brake fluid.
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Old 05-07-2015, 07:05 AM   #10
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I was referencing parts that are seldom an issue. The proportional valve, while it is an integral parts of every brake system they almost never cause any problems. They require no regular service or maintenance and therefore are almost never on the technicians radar. In this instance both front brakes would apply but would not release. The MC has already been replaced. Using common sense and reasoning (diagnosis) there was only one part that was common to both front brakes. Both front brake hoses had already been replaced and the front line leaving the PV was common to both front wheel cylinders. The only possible source of the no pressure release had to be a plugged PV and it was. Simply applying reverse air pressure blew a small piece of rubber hose out of the valve. Problem that I had never experienced was solved.

You are loosing fluid and it's not external so it can only be going into one place the brake vacuum booster. Common sense and deductive reasoning point to it.

We are doing a diagnosis from a great distance and poster #8 asked about rear drums. Many are assuming (we all know what that can get us) that's been checked. We could be wrong and poster #8 could be correct. If it's not been checked then that may be your source of the leak. If you have drums they need to be pulled and check the wheel cylinders.

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Old 05-07-2015, 09:14 AM   #11
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Thanks all. Just heard back from mechanics that we have a hydro boost system, not vacuum boost. Don't know if that percolates any other ideas of things to check.
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Old 05-07-2015, 08:29 PM   #12
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Well then you will need to find the source of the leak in the lines, calipers or wheel cylinders.
it should be somewhat obvious as you have described a pretty rapid leak.
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Old 05-07-2015, 10:12 PM   #13
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We got the RV back. They replaced the master cylinder and power flushed the lines. Inspected lines and the 6 disk brakes. No leaks were found. They thought the old master cylinder did not look newer so maybe we got ripped off last time. We've had this shop do work before and they seem reliable.

Since all three times it has only happened after we put the RV to bed in the winter (and had arctic freezes) we may not know more until late in the year.

Appreciate everyone's help.
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Old 05-07-2015, 10:28 PM   #14
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I also have the Hydroboost. Not many mechanics are familiar with the system. The mobile mechanic found that the bolts that held the 2 systems together were loose and it was leaking brake fluid. Hydroboost uses the Power Steering system to increase braking.

*** ALSO *** Do you have a tag axle? Sounds like you do. ???
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