Strange Brake Problem 85 380SL - Mercedes-Benz Forum
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#1 (permalink) Old 06-14-2010, 07:03 PM
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Strange Brake Problem 85 380SL

OK...quick seniro: Driving the car last week.went to stop and the pedal went to the floor. A few quick pumps and the brakes held but very little pedal.
Made it home and checked the fluid. The fluid level was correct no more , no less. Check all lines and connections for any sign of a leak or problem.Checked all rubber hoses for any bulging...NOTHING
So i have since replaced both front calipers, rubber hoses,pads,pad sensors, and rotors as well as new front wheel bearings and new master cylinder.
All lines were bled but still cant get more than one inch of pedal before hitting the floor. I have rebled the entire system using a mighty vac pump and also by using the old method of manually pumping the brake with a helper. Still the same...no pedal.
The weird thing is that with the car switched off (not running), I can get a full firm pedal that holds without sinking. But when i start the car, i cannot get a firm pedal at all, and if it does seem to firm up after a few pumps, then it sinks to the floor pretty quick. I have bled the brakes on this car many times and flushed the fluid as well as many other cars and never come across this, has anyone else experienced this? Am i missing something?
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#2 (permalink) Old 06-14-2010, 08:56 PM
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I'm no car expert, but it sounds a lot like the master cylinder I had to replace on my last convertible before I could sell it. It was sitting for a while, and pulled it out of the garage to find a similar problem. I think it was around $500 for the master cylinder to be replaced in that car. (Too embarrassed to say the type of convertible it was because it was so ugly.)
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#3 (permalink) Old 06-14-2010, 09:03 PM
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Change the booster valve, it's on the vacuum line off the booster. It's cheep and more than likley the problem.
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#4 (permalink) Old 06-14-2010, 09:13 PM
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From your info, I think you have ABS brake system. I seem to remember that there are some extra, specific steps for bleeding those systems. I just got through with a master cylinder replacement due to the same exact symptom, but mine predates the ABS.

I had trouble trying to bleed mine with a MightyVac, and pulled out the $20.00 Do It Yourself Pressure Bleeder. Perfect, fast one-man sytem bleed.

I'm not up on the ABS brakes, but I seem to remember someone saying that theirs needed to be pressure bled, and there was someone who had to go to the dealer to have something reset / reprogrammed / calibrated on theirs.

Good luck,
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#5 (permalink) Old 06-14-2010, 09:45 PM
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Just to clarify, the car in which I experienced a similar situation had ABS. (and I was told it was the master cylinder, and I had it replaced for ~$500 including labor at the Infiniti dealer and the problem was fixed. Oh did I say Infiniti.) I think the explanation was simply letting the rubber parts in the master cylinder dry out from lack of use. So when I started driving it again it the problem was sitting there waiting for me.

Bleeding the lines seems important, but I have to ask... what would have introduced air into the lines? (I'm assuming it was fine before.) Isn't air typically only introduced when the fluid gets low, or the brake work is done?
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#6 (permalink) Old 06-14-2010, 09:57 PM
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Usually it is not air that causes a master to fail, non-use, wrong fluid when topping off are the usual culprits. Dot-3 fluid (which is not used on theses cars), absorbs moisture and can cause corrosion from the inside out on the brake lines. This is why it is important to flush the lines.
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#7 (permalink) Old 06-15-2010, 06:07 AM
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I just bought mine a few days ago. If they don't use dot3, What kind of brake fluid do "these cars" use? (Mine is a 1973 450 SL.)
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#8 (permalink) Old 06-15-2010, 06:30 AM
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#9 (permalink) Old 06-15-2010, 09:01 AM
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if i recall, you need to turn the ignition to ON to cycle the ABS pump while you bleed it.
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#10 (permalink) Old 10-16-2010, 02:31 PM
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strange braking problem 380 SL

I have exactly the same issue - brake pedal to the floor when the engine is on, have also replaced the master cylinder and the problem persists.

I've now got an added problem the car also judders (so much so I thought I had a flat tyre)

did you get any useful resolution to your issue - one suggestion was to replace the booster valve - did this cure the problem?
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