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Low Brake Pedal

Old 02-16-2007, 01:27 AM
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Join Date: Feb 2007
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Default Low Brake Pedal


My mother drives an 86' 420 SEL, and just recently the brake pedal drops to the floor and with little effort to brake. There is plenty of fluid in the reservoir, and the Red Warning lamp did not illuminate letting me know if there was a problem with system.

Now I am fluent in most conventional brake systems, but I don't know Mercedes hydraulic brake systems too well. The front rotors and pads, I replaced those a year ago, due to the fact that the run out was bad from sitting for awhile two years ago before getting the car. I am familiar somewhat with the Bosch ABS Unit, but doubt that is issue that would cause a low pedal, that drops to the floor. Unless in the ABS unit it self the dump valve on two or more wheels has stuck open allowing fluid to flow into the reserve chambers and causing the pedal to drop, but on the other hand, this would also cause the fluid level to drop in the reservoir, which it hasen't.

Where do I go from here? Could it be as simple as bleeding the brakes? Anyone else experience this before?

Thanks again

Old 02-17-2007, 01:06 AM
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Join Date: Jan 2006
Location: Maryland
Posts: 1,561
Default RE: Low Brake Pedal

brake pedal drops to the floor
I had the same problem and my mechanic simply flushed and refilled the lines.
Old 02-17-2007, 03:39 AM
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Join Date: Feb 2007
Posts: 2
Default RE: Low Brake Pedal

Well, after looking over and over again, it seemed that it was the master cylinder at fault. And another guy I know mentioned that his had the same problem as I described, and he said the primary piston in the master cylinder; its primary seal went out and made it useless for the system to create pressure.

But, since the car is 21 years old and probably hasen't had the brakes bled or flushed, it probably needs this done anyways. I will probably do this in addition to the new master cylinder tomorrow.
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