Brakes - Help - Mercedes-Benz Forum

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post #1 of 4 (permalink) Old 08-04-2011, 04:26 PM Thread Starter
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Brakes - Help

I've changed the brake booster on a '68 280 S, used the correct booster and bled the brakes at the wheels. The master cylinder is full. The pedal is firm and there is no vacuum loss sound (hissing air) or change in idle when I apply the brakes.

The problem is that the car seems to require too much pedal pressure now.

I'm concerned that I might be braking on only two whelels. Is the new (used) booster suspect?

Could this be caused by incorrect bleeding?

Thanks for the help.
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post #2 of 4 (permalink) Old 08-05-2011, 04:31 AM
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I can tell you how to test a booster...
- with the car off pump it until the vacuum dissipates (usually 2-4 pumps) the pedal should be very firm. (this will tell you if there are any major leaks, a leaky booster will never become firm)
- holding the pedal, start the car, you should feel the pedal drop 3"-4" (This verifies a presence of vacuum in the booster, if the booster is not receiving vacuum pressure the pedal will remain firm and not drop)

After that go from there...
-Incorrect bleeding usually results in a "squishy" or soft brake pedal that has no definite stop to it.
proper bleeding is done with the car off and watching the brake fluid reservoir making sure to not run out of fluid and add air to the system:
(2 person): press brake pedal and hold, have other person open bleeder until bubbles/fluid stops flowing, close bleeder, lift foot off brake pedal and repeat until clean fluid flows out of bleeder with no bubbles in it.
(1 person) attach a hose to the bleeder submerge end of hose in a cup half full of brake fluid, open bleeder, pump brake pedal until clean fluid comes out bubble free, close bleeder.

I've always preferred 2 person as it is less messy and in my opinion easier to decide when the fluid is clean, but they sell one man bleeder kits in all the autoparts stores, they also sell one-way bleeder valves for cars, that make bleeding brakes very easy, but you probably wouldn't find any that fit a 1968 Mercedes.

-other possibilities include collapsed brake hoses on the calipers and rear axle preventing the fluid from moving, but this usually results in rubbing brakes as they are not disengaging.

Last edited by tgikal; 08-05-2011 at 04:45 AM.
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post #3 of 4 (permalink) Old 08-05-2011, 06:55 AM
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I had the same issue until the front hoses were replaced,
Internal collapse (swelling closed).

The visible clue was a lack of volume when bleeding front calipers.

Brian O
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post #4 of 4 (permalink) Old 08-05-2011, 08:34 AM Thread Starter
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Thanks. Good advice.
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