break booster or master cylinder - PeachParts Mercedes-Benz Forum

js1sj · #1 10-18-2006, 08:05 PM

the fromt breaks on my W123 are bressing hard on the rotors, im thinking somthing is off with the master cylinder or the break booster,
any thoughts

sixto · #2 10-18-2006, 08:39 PM

I don't understand the question.

I would hope the brakes press hard because it takes a lot of force to stop a car. Rear pads usually last 2-3 times as long as front pads so different rate of wear front and rear is not necessarily an indication of a problem.

Does your car have a proportioning valve?

Sixto
93 300SD

Larry Delor · #3 10-18-2006, 08:39 PM

What do you mean by they are pressing hard on the rotors?
Do you mean, you need to press hard on the brake pedal to stop? - If this is the case, then it could be that you have a vacuum leak, and the booster isn't able to help.

Please explain what you mean.

js1sj · #4 10-19-2006, 09:59 AM

even when the break peddle is not pressed , the brakes are on, the break calipers are fine and the pads are good, i blead the breaks inorder to try to solve the problem , but once i drove the car the breaks tighented back up, the car is not drivable as the front breaks press so hard as to not let the car go up hill. with the front of the car jacked up, it is almost impossible to turn the front tires by hand.
Cheers

DustyRusty · #5 10-19-2006, 10:05 AM

Start by replacing the rubber hoses if they have never been replaced in the past. If the hoses are old, they might not be letting the fluid return to the master cylinder. Then bleed and see if the problem continues. If it does, then there is a problem in the proportioning valve or booster. Most likely the booster is applying pressure because of a torn diaphragm. Is this a US or Euro version car???

js1sj · #6 10-19-2006, 10:08 AM

the car is a 1984 300d turbodiesel W/ 200,000, origionaly sold in las vegas

DustyRusty · #7 10-19-2006, 10:25 AM

With the front jacked up, try turning both wheels. If both are locked up, then bleed the right side of the car by opening the brake bleeder. DO NOT touch the brake pedal. Just open the bleed screw and allow the fluid to come out. Close the bleeder, and then check the other wheel to see if it is still locked. If it is, then the hoses are bad. If it isn't, then the problem is upstream. Have an assistant help you bleed the brake that you had just opened the bleed screw. Then step on the brake once and see if both wheels are locked. If they are, then you need to loosen the brake master cylinder from the booster. If the wheels then are able to be moved, the booster is the problem. If they are still locked up and the master cylinder is loose from the booster, it is the master cylinder or proportioning valve. I know of no easy way to test a proportioning valve, so you will be on your own to decide which one you want to replace for trial and error method of repairing. You also might have more than one problem with the system that will require replacement of two or three items to resolve the issue.

david s poole · #8 10-19-2006, 03:07 PM

i have replaced a lot of master cylinders on that model for your exact problem.there is a valve that jams in the m/c causing brakes to stay on.