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 car does not pull to either direction while cruising or braking. This happens only after the engine warms up and doesn't seem to be related to how much or hard I use the brakes. For example, I can have a perfectly normal brake pedal with 1-2" of free travel. I'll climb a hill and when I apply them again, there is no free travel what so ever and now I'm really getting on the gas just to go. I can pull up to a gas station, pedal feel normal, shut the car off for say 5 minutes, as soon as I get back in, the pedal is stiff with no free play. However, if it's cold out side and I'm moving, I can use the brakes quite a bit and the pedal feel stays the same, some free play, and there are no signs of dragging. At this point I'm hypothisizing that when the brake fluid heats up, it's not bleeding back into the master cylinder for some reason. Before adding a second pedal spring, I used to be able to occasionally relieve this by raising the pedal with my foot. However, that no longer works as the second spring ensures it's at the top of its travel.



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

While I think I was seeing to problems, the original return spring going soft and what ever problem I have now, I now suspect my current problem has something to do with the master cylinder or something not alowing fluid to bleed back into the ms when the car warms up.

Any insights would be appreciated.


Frank.
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'79 300D.

I would check the free play in the pedal between the pedal and the master cylinder. There must be some free play where the master cylinder can return completely to receive and distribute fluid.

Brake Drag
 

Check your power booster and free play on your master cylinder to booster. Check your pedal free play on pedalto booster. There is also a distributor to the back wheels, not sure if this can cause this problem. Have you checked to see if all wheels or just front or rears are dragging?

Partsman

I didn't check all the wheels, just the fronts. I'll do that.

As for the free play, when it's cool outside, or the car is off (and not hot) there is plenty of free play. It's after the engine warms up and, say I pull up to a light, the free play starts to go away. If it's hot out, or I'm climbing a hill, basically doing something that gets the motor/engine compartment good and hot, the free play is gone and the brakes are draggin' good. That's what is weird about this. Any car is going to have brake lines that warm up and fluid that is expanding, as far as I can tell, this fluid goes back into the master cylinder when the brakes aren't engaged. That just doesn't seem to happen on this car. Instead, the pressure builds and engages the brakes.

Frank.

Let me share an odd experience of mine that may be related to your problem.

A number of years ago I headed to the office early one morning in my '84 240D(same chassis as yours). My office is 90 miles from the house and it was still dark. I was about 15 miles from the house and thought the engine was losing power. I slowed to get on the uphill ramp to the highway and the car wouldn't accelerate up the ramp.

I stopped, got out and started walking around the car. I immediately noticed the left rear brake rotor was RED HOT. It looked like a red neon sign. I walked to the other side and that rotor was glowing as well.

I was lucky enough to be where I had a car phone signal, and called the wife to come get me. By the time she got there, I had found and cured the problem. I got an open end out of the toolbox and put it on the master cylinder fitting for the rear brakes. I put a rag over it and cracked the fitting open. I felt fluid shoot out over my hand. There was extreme pressure there, and noone had their foot on the brake. I bought a master / cylinder that day and pitched it in the trunk. I went a long time after that with no more problems and some time later when doing a brake job, I put on the new master cylinder, after all I had already paid for it and it was there.

I can only assume that somehow the master cylinder did this, but I'll never know for sure.


BTW, with that said, don't overlook the fact that the emergency brake consists of shoes inside the rear, outer rotors that are actuated by a mechanical mechanism.

Best of luck,