I got in the car and the brake pedal won't move, and the brakes won't work. I let the car warm up and shut it off and same thing happened. I left it all day and it got above freezing (40 degrees), came home started the car and the pedal was firm then got softer and seemed sluggishly normal, I moved the car and it felt better, so I moved some more and drove. The brakes worked fine and then the problem came back after a little pumping. I had water get in the booster/master cylinder area after I had the fluid flushed. I tracked the car in Nov with fresh fluid with no problems. I got the water in before that. I drove the car last Sat night and everything was fine, didn't drive until Tuesday with three days below freezing, and now have this problem.
#1 Could moisture be in the booster and caused it to freeze?
My first thought is to get the system flushed Monday and go from there, any suggestions?
#2 Could moisture in the brake lines freeze and not allow fluid to move producing a hard pedal?
I'm not sure if the M5 is the same as my wifes E39 530i (I'm just a prospective M5 owner), but in her E39 the brake booster is beneath the cabin fresh air intake box on the driver's side of the engine compartment. Remove that box and you will see the booster encased in a box of its own. This box can fill up with water when it rains if the 0.0000000000291 millimeter drain hole at the bottom (conveniently difficult to get to as it is under the booster) gets clogged up with debris. In my case that debris was decayed leaves, and one rainy night the brake pedal got hard and when pushed I got a squishy sound. Somehow the water had gotten into the booster itself (not sure how really). It ultimately stalled the engine and wouldn't start as it sucked water through the brake booster vacuum line. I had it towed home and was lucky enough to be able to remove the spark plugs, blow out the cylinders with some compressed air and all was good (I used the compressed air to blow out the drain hole too. If you got water in there and temps were below freezing it could have frozen...and then later thawed, etc.
My bet is on water in the booster too. Had this happen on an old style VW Passat, blocked drains. Booster had about 0.25L water in it, which would freeze at below zero temps, causing a rock hard pedal. Only thing to do about it is to replace the booster as the water attacks the metal housing, plate and spring inside) and include cleaning that drain plug mentioned above in your regular maintenance. I believe it's mentioned in the TIS too.
Thanks guys, I am getting it to my indy tommorrow. I am going to see what they say since that cavity filled with water after they worked in that area. The water was dripping straight into it and not going out of the drain. We had a but load of rain last month here in AL (ROLL TIDE), and I got water in the back seat floor. This is how I found the source of the water leak. I would say at least 8 inches of water in that area around the booster. Great design to allow water in so easily with a drain hole the size of a pin! At least the rest of the car is well engineered and produces many grins!