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Old 12-30-2003, 04:45 AM
zeronero's Avatar
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Location: San Francisco Bay Area
Posts: 426
Brakes going bad, could be master cylinder or booster, but which?

A while ago I put up a post about my brakes and the consensus was that it is the master cylinder that is bad. I was told that it could also be the booster on the other hand from other sources. So I'll tell you what the situation is and you can tell me your thoughts.

All the pads on my car a fine and the rotors are as well, the fronts rotors are almost up for replacement but not yet. There are no visible leaks in the system at the calipers and master cylinder and the fluid level has always stayed the same. The brakes have not been bled for at least a year. ABS works when it needs to. Pedal pressure is the same as its always been.

What happens is that at a stoplight the pedal slowly loses pressure and the car creeps foward. If I immediately pump the pedal it gets firm quickly and will normally hold, only sometimes will it lose pressure again. The pedal always feels firm, just looses pressure and falls to the floor.

From the search and reading past posts I'm fairly confident that it is the master cylinder, I just want to be sure. The only thing that I have heard about the booster going bad is that it creates a spongy feel which I don't have. So any thoughts?
1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500
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Old 12-30-2003, 05:46 AM
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Join Date: Jul 2003
Location: Atlanta GA
Posts: 16

With the engine off pump the brake pedal several times until it feel firm, holding your foot down firmly on the brake pedal start the engine, the pedal should move in as the engine starts.
If it does the brake booster is working.
The master cylinder could be leaking internally past it's seals.

1995 Mercedes E320
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Old 12-30-2003, 09:40 AM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
I agree. Sounds like a new master cylinder is in your future.

I hope you're not driving this car until it gets fixed ...
'93 400E
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Old 12-30-2003, 12:23 PM
csnow's Avatar
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Location: Mass
Posts: 1,127
I think I voted before, but I will vote for the MC again if it makes you feel better.

A booster problem is not going to let the pedal fall to the floor and a loss of system pressure. It would just increase the leg force required to accomplish pressure.
1986 300E 5-Speed 240k mi.
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Old 12-30-2003, 03:54 PM
87-300E_in_NC's Avatar
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Join Date: Dec 2003
Location: outside of Raleigh, NC
Posts: 203
Wink Master Cylinder 4-sure

Master cylinder 4 sure.

I experienced the same situation in another car while on a drive to New York, and before I could get there, I was losing brake pedal pressure on the NJ turnpike! Not a good feeling, so I had to gear down and use the hand brake until I could get to an auto parts store, and then a hotel where I could change it out in the parking lot.

When I removed the master cylinder, I discovered where my fluid had gone....into the brake booster, where the rubber diaphram is, and I knew that I had to clean it completely (if not the brake fluid would destroy the diaphram). So, thinking of what to use in a hotel parking lot at night, I looked in one tool bag, and found my fuel pressure guage....I removed the overflow tube (small clear tube) and hooked it to a vacum on my intake of the engine.

I then placed the tube in the booster, cranked the engine for vacum, and let the engine suck the brake fluid out. Needless to say, the white smoke at the tailpipe was clouding up inthe parking lot, since it was buring in the exhaust. When I was done, I tried to clear the smoke up, but was tired, so I went to sleep, and got up early the next morning, to get back on the NJ turnpike. Everyone was turning their heads while me and my girlfriend traveled on the highway, since we were leaving a thick white smoke trail behind us for miles. My girlfriend asked if it would ever quit smoking.....I laughed and said yes, eventually.

It smoked for almost 20 miles before buring away! Drivers on the road gave me plenty of space, and they caught up to me after it quit smoking (the ones that tried to motion t me that I was smoking )

Hope this helps. Good luck.
'87 300E | 2.6 engine
Artic White - Navy blue interior, chrome rims, very clean and sharp!
91 300E
89 300E

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Old 12-30-2003, 09:06 PM
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Posts: 305
Yet another MC vote... Internal leakage..
89 300SEL Money Pit

92 Blown Buick Ultra Pimpmobile 220K and adding 1K per week

88 Wagoneer Slightly modified (Not for soccer moms)

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Old 12-31-2003, 12:02 AM
zeronero's Avatar
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Location: San Francisco Bay Area
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Since everyone agrees that it's the master cylinder I went out today and bought one along with all the parts that are required to do the 150k service. I got a good deal on the master cylinder, it was less than $100. I think that I'm going to redo the whole brake system while I'm at it. Thanks for the info.
1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500
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Old 12-31-2003, 07:09 AM
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Join Date: May 2000
Location: Soperton, Ga. USA
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Don't forget to bench bleed the m/c prior to installation.
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Old 12-31-2003, 08:09 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Master cylinder.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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