My 560SL has developed a scary problem with the brakes. Normally when I step on the brakes, the peddle feels firm and only goes down an inch or 2 and brings the car to a smooth even stop. Lately the brakes seem to have a mind of their own and sometimes the peddle will go almost all the way to the floor before it builds up pressure. It always stops OK but I never know if it's going to catch at the top or bottom of the range. There doesn't seem to be a pattern to when this will happen. Sometimes when I first start the car , sometimes not until I have driven a while, and it will switch back and forth while driving in traffic. When the peddle goes to the floor, I have tried pumping the brakes to see if it will build up pressure but it doesn't make a difference. When they are firm at the top of the range I have tried to keep applying a lot of pressure with my foot to see if there is any leak down and there is not. .... Master Cylinder??Brake Booster??? Vaccume leak??? Any Ideas Thanks in advance...

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Hud

88 560SL
97 E320

My first guess would be the master cylinder is the culprit.

There could be air in the lines if they were not bled properly, but I would think that if continued pumping of the brakes does not build pressure then this would indicate the master cylinder is unable to build pressure.

If it were a problem with the booster or a vacuum leak to the booster, the pedal would not got to the floor, you would just have to push the pedal really hard to stop the car, but pedal travel would not increase.

Assuming no brake fluid leaks, no air in the line, proper level of fluid in the reservoir, etc, I would be inclined to believe the master cylinder is at fault.

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record

Master cylinder

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Jeff Lawrence
1989 300e
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.

Master cylinder, without a doubt.

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I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

Wow! we have 3 for master cylinder, and that is where I was leaning.. I guess I'll be ordering a master cylinder in the next few days. Thanks for the sanity check guy's

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Hud

88 560SL
97 E320

HI Jerry

I'm with the others on this one, Ive seen master cylinders do this very same thing. But I want to ask you a question. When the brake pedal is low do you feel any vibration or buzzzing in the brake pedal? I'm wondring about ABS problems is why I am asking. It will give you a low sinking pedal feel if it is not function properly. Just hate to see you buy a very expensive master cylinder when it may not be the problem. Brake problems are something not to be taken lightly. Make sure you let us know the fix or if you need more help.

fasthair

Has anyone suggested master cylinder?

I will be shocked if replacing the master cylinder and properly bleeding the system does not make this problem disappear.

Have a great day,

HI Larry

That is why I asked these questions. I have my doubts it's the master too but want to get all the info I can to make an infromed conculsion.

fasthair

Had the same problem with my 380sl. The pedal was firm at startup and a couple of stops. Usually after 2 or 3 stops, the pedal would go all the way to the floor. I could hear a whooshing sound under the dash. If you can still get braking if you push all the way down, I'd bet it's the vaccuum booster (behind the master cylinder). I learned the hard way by having the MC replaced, only to drive off and have no brakes after spending $150 for a new MC I didn't need.

Vacuum boosters are cheap to buy (used less than $50) but a pain the the butt to install so I hope for your sake it's the MC. Check the manifold vacuum hose first.

Good luck.

J. Boggs

Thanks for the additional input. Fasthair: no there is no vibration or pulsating the would indicate ABS failure. J Boggs: I dont hear any wooshing sound. When this occures, the pedal either gets firm close to the top of travel or it gets firm about 2/3 of the way down. There is never a sense that the brakes won't stop the car, it's just where in the total range of travel will they become firm..

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Hud

88 560SL
97 E320

HI Jerry

If there is no feeling like the ABS is coming on to soon or when not needed then I would go with the master cylinder myself. Seen many cause this very same complaint. Sounds like it is intermiantly by-passing one of the seals in the master. Mr. Boggs whooshing/hissing sound would indicate a leaking brake booster for sure which is normaly accompied by a high and hard pedal that takes more effort then normanl to stop the car.

fasthair

Thanks for everyones help. I installed a new master cylinder and new pads all around and the brakes are working great. It was odd though because the old master cylinder looked new. Thanks again..............

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Hud

88 560SL
97 E320