79 300D Brake dragging
 

Here's the deal. My car's brakes drag. The symptoms are high pitched squeeling when driving down the road but no foot pressure on the pedal. The car just won't go, again no pressure. The pedal sometimes gets very firm and engages as soon as you press it when the above things are happening. (as opposed to there being an inch or so of play like there is when the brakes don't exhibit these symtoms) I used to rememdy this by simply pulling up on the brake pedal. I could actually lift it and the brakes would ease up. (even jacked the car up, could bearly turn the wheel, pulled on the brake pedal, wheel freed up. That car was off when I did this) However this stopped working. I even installed another spring to keep the pedal up and the problem is still there.

The parts that have been replaced/rebuilt in chasing this are

Brake hoses
rebuilt calipers numerous times
new master cylinder
system bled and flushed with many bottles of brake fluid
An extra return spring installed on the brake pedal

According to the previous owner, my dad, the only original untouched part of the brake system is the booster. He said that when one of the master cylinders went out, it leaked into the booster. I don't know if that could have anything to do with this, but it's a little more info.

Anyway, this problem has been driving my dad and I nuts for quite a long time. If anyone has any ideas, we'd be super stoked.

Thanks

Frank.

Frank,

Did you rebuild the brake calipers or did you buy rebuilt ones? In any case I suspect your problem is the brake caliper cylinder bore to piston seal is damaged. The way disc brakes work, when you step on the pedal the pressure from the master cylinder pushes the cylinder in the caliper out of its bore and against the pad, which bears on the disc. When you take your foot off the pedal, the spring return on the pedal acts to take the weight of the pedal off the master cylinder, but that does not create a vacuum to pull the caliper piston back, as any such unfilled volume is filled with brake fluid from the reservoir. Believe it or not, what releases the disc is the little square cross section "O"-ring seal. It is supposed to be in intimate contact (no lubricant other than the brake fluid) with the piston outside diameter, and has a high coefficient of friction. As the brake is applied, the outward motion of the piston stretches the seal, and generates a preload pushing it back in. When you take your foot off the pedal, and the pedal returns to its normal position of not applying any load to the master cylinder, the seal preload pushes the piston back slightly. That slight motion allows the pad to let go of the disc. If this does not occur, the whole brake stays "on" and then gets real hot, which causes more force to be applied, and so on.

While other events are possible, the most common cause of such a problem is the seal. When the caliper is rebuilt the geometry of the seal cavity and the clearances are critical, as is cleanliness. Hope this helps. Jim

Jim, thanks for the reply. I think I'm going to look into some new calipers. These are 22 years old and have 350K miles on them. While they have been rebuilt, maybe something is worn to far. Either way, they sure have done their time on this car!

Frank.

Frank,

I would agree with your decision, that is a whole lot of life out of a set of brake calipers. Good luck and hope you get another 350,000 miles!

Jim

be sure to get ate pads if you get rebuilt ate calipers, I bought my rebuilt ate calipers at Kenects before I found this site they were loaded with cheapo pads ,and the brakes squealed like the proverbial stuck pig, next tried NAPA pads same problem. Then I found this site and Fast lane installed new master cylinder and ate pad all the way around what a diference.Faslane and parts shop are a true blessing to this double MB owner....
William Rogers.....