Hi group, my 1985 GL has presented me with a scary problem. I use an indie
mechanic who really does not know the cause nor the solution. Problem: brake pedal goes straight down to floorboard without warning. We switched-out master cylinder 3 times; problem was less frequent but not solved. The last quest was
to clean and reinstall the small plastic piece that chumsup to brake booster unit. That helped alot and last winter/fall/spring operation was ok. The first
really hot day this summer revisited the problem and I was lucky not to have had an accident. The problem is continuing in this +-90 degree weather and I am
reluctant to drive my revered 240. Anyone had this same problem? Thanks in advance for any help you can offer, cheers, Damon

If a pad is dragging enough to boil the fluid, you should be able to feel the drag by jacking up each wheel and turning it by hand. A piston might be stuck and not retracting into the caliper. When new pads are installed, the pistons are pushed back into the calipers farther than before. A piston may have gotten cocked and jammed or stuck in the deposits at the back of the caliper. So, the dragging and overheating is just a symptom of a piston problem.

Thank you very much. This is the best information I have received.

So when this happens do the brakes "recover" in time? Does it only happen when the car has been driven for a while?
If one of the front calipers is dragging the fluid can boil in both circuits and cause the problem and when it cools return the brakes to near normal. Check for hot brakes on one side or the other.
If the master cylinders are reman and not new, and are failing about once a year, that's about right. I don't use reman at all. New only.
Other poster mentioned this and was 100% correct that the booster unit has nothing to do with the problem. If your mechanic thinks it does change mechanics.

Flathood thanks for your ideas. Yes, happens when car has been driven for some
time. I believe the hot-brake is the problem and I will show your msg to my
mechanic. The master cylinder I bought new from BAP/GEON, $112 Volvo-Sweden part, so not a reman. Take care; this was a very good post.

If one of the front calipers is dragging the fluid can boil in both circuits and cause the problem and when it cools return the brakes to near normal. Check for hot brakes on one side or the other.
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As Peetz said

I did not think of this, since you said yr mechanic ....etc

This I have experienced, not on a Volvo, but the same principle, on a Honda,

The brake fluid "boiled" actually going uphill...

when I lossened one of the bleeders it shot out brake fluid...yikes misssed me EYE..glasses.

It was an interesting trip back down the hill, with my Volvo holding back the brakeless Honda on my bumper

only for a friend.

Thanks for your interesting posts; will show them to my mechanic.

A brake booster problem would not cause the pedal to "go to the floor".

Lack of vaccum boost assist, would cause instead a hard pedal.

Because your Volvo has a seperate/dual circuit braking system,\ even it you blow out brake pressure on one side of the system with major Leak, you will still have pressure on the other side of the system.

really the only component that would cause the "to the floor" pedal would be a failure of the Master cylinder. You have lost hydralic pressure "catastrophically" on both sides of the dual circuit.

Any other cause would be obvious, as there would be fresh brake fluid residue dripping at the site of the Big Leak. Which your mechanic should be able to easily see.

When the pedal goes to the floor, do you have any braking at all ?
Have you had any brake work done other than changing the master cylinder ?
Have you tried a different brand of master cylinder ?

Greg

Greg thanks: Have to use my hand-brake when it happens. Replaced pads. Master
cylinder was not a reman; it was a $112. Volvo-Sweden new one.
I believe I have a "hot" brake and am going to check it out. Appreciate your
good questions. Damon