Need Tech help on scary Brake problem
 

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...

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.

Master cylinder

Master cylinder, without a doubt.

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

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

Probably MC but check vacuum booster.
 

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..

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..............