I posted awhile ago about my intermittent dragging brakes on my 1988 740 turbo sw. Some of you may remember the little squabble that erupted about me being a troll, perhaps because of my phraseology; I don't really remember. So now all is forgiven and I have FINALLY found a competent mechanic who was willing to drive my baby around awhile, until the brakes began to drag. Both front calipers frozen YAY, a diagnosis! BUT, if this $400. repair doesn't fix the problem. I'm looking at replacing the master cylinder, which is a lot more $$. They say they can't tell if it's the mc until those calipers are replaced.
So, my question is, how does a master cylinder work, and what else might have caused both of these calipers to fail? And so intermittently! It's taken me almost a year to get the convergence of good mechanic looking at car WHEN the brakes were acting up.
Any explanations or feedback welcome; just don't call me that "T" word again!

In my experience a "frozen" caliper means that the piston will no longer move- as in seized in place. A seized caliper is not an intermittent condition.

As was suggested the rubber brake hoses are suspect in the problem you describe. It sounds like they are breaking down internally. Applying the brakes creates enough pressure to push past blockage in the hose. Releasing the brakes is not releasing the pressure in the calipers and causing the pads to drag on the rotors.

If your experience is the brakes drag after driving the car for awhile and then seem to release after sitting for awhile ask your mechanic how that results in giving a diagnosis of "frozen calipers".

Randy
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Any twenty minute job is just a broken bolt away from a three day ordeal

Hi Randy,

I think it may be possible for the caliper to be at fault without having a seized piston.

A few years ago I had a dragging rear brake on a 240. One of the pistons was difficult to retract and I assumed it was in the beginning stages of a seizure.

When I split the caliper open to rebuild it, I found a snotty brown semi solid mass stuck at the junction where the fluid goes from one side of the caliper to the other. I fished the lump out with a dental pick, put the caliper back together and it's been fine ever since. The car was new to me so a double flush of the system soon followed the repair.

regards, Peter

Peter,

That is interesting. Do you remember if the piston would retract a bit after sitting for a while? Perhaps I am interpreting the post wrong but it seems his brakes would drag after driving for a while and then evidently retract some before he once again drove the car only to find the brakes starting to drag again.

I was under the impression that once a caliper is split it is difficult to prevent them from leaking. Are there O-rings between the halves and did you replace those?

Randy
--
Any twenty minute job is just a broken bolt away from a three day ordeal

Hi Randy,

I came across this condition while hunting down an intermittent noise that seemed to be wheel related. After a drive I found one rear rotor to be warmer than the rest, although not blistering hot as a seized piston might produce. A few weeks after the repair the noise got worse and it became obvious that the culprit was a front wheel bearing.

The piston would retract every time, but only to a certain point and it took more effort than is normal to spin the wheel. I think the blockage was acting as a check valve. It may well have released the pressure over time but I don't know because I pulled the caliper as soon as I found it. The pad was not showing excessive wear.

Both Gerling and ATE calipers have O rings of a rectangular section between the halves. Sometimes they come with the kits and sometimes not. I usually save the new ones and reuse the old unless they're visibly compromised, and have had no leaks in a couple of dozen rebuilds.

regards, Peter

If the rubber lines going to the calipers show age, or if you're unsure of their age, I would also have those lines replaced at the same time as the calipers.

I tried to look up your previous post for more information regarding this ongoing issue, but didn't turn anything up. I apologize if this has already been done or recommended.