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Old 11-12-2001, 10:33 AM
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low brake pedal/brake pedal creep

Just bought a 1983 SD with brake problems. I have put on new pads, new fluid, new master cylinder (the old one felt just like the new one - also the booster unit does not look that old). Have bled the brakes. When stepping on brakes with engine off, or master cylinder plugged everything feels good. When stepping on the brakes for the first time in an application, with the engine running, pedal is low. with one pump, the pedal comes up but creeps under steady moderate pressure (it seems to creep less with greater pressure). I lose no fluid, and have no puddles. The stopping performance is not so well as with my 82 240D, but the vacuum readings I take are similar to those on the 240D. Is there something to bleed or adjust besides callipers and master cylinder (a proportioning valve or similar?)

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Old 11-12-2001, 01:35 PM
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Join Date: Apr 2000
Location: So. Cal
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Did you install a new or reman'd master cylinder?

Still sounds like a bad master cylinder to me. Also, did you bench bleed the master before installing? I have read that the MB masters need to be bench bled prior to installation to ensure proper performance.
Mike Tangas
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Old 11-12-2001, 08:15 PM
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Pedal creep

Yes I did bench bleed the master cylinder, which was new, (ATE).
I just now noticed that the pedal creep accelerates as I increase the engine speed -- seems to increase as the vacuum pump pumps more. Could I have a restricted air intake on the atmosphere side of the power booster, or could my vacuum pump, even though it reaches the same pressure as that in my 240D be pumping less volume?
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Old 11-12-2001, 08:37 PM
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Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
You may have a ruptured diaphram in the booster. Does it hold vacuum with the engine off? You should be able to apply the brakes with full boost a couple times before the vacuum all gone -- if you don't have at least one application's worth of vacuum in the booster after you shut the engine off, it is shot.

Poor braking could also be air somewhere else, too -- bleed the whole system to replace the fluid if you haven't done so.

If you have a high, hard pedal without vacuum, there probably isn't anything wrong with they hydraulics, unless you don't have the seal between the master cylinder and the brake booster.


1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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