Brake fluid boiling should never occur on a street driven machine unless something is wrong. It takes extreme use, such as on a racetrack, or perhaps descending a long mountainside using the brakes the entire way, to cause fluid boiling and even there it's fairly rare. I've done hundreds of laps of racing events and track day driving in various machines and only once experienced pressure loss due to fluid boiling, and that was in a 6 hour enduro race.
If a M3W is boiling the fluid in normal street driving then one of the brakes must be dragging. Easy to check as when it occurs you can do a simple temperature check with your hand to find the offending brake as it will be extremely hot. Most likely the culprit is the rear brake, skulking as it does in the dark recesses of the back end where no air flows. On my car at least, the back brake seems to be somewhat variable in the amount of drag it evidences. Pushing my machine around in the garage I've noticed that if I push in reverse for some distance (like 20ft +) the rear brake will sometimes lock up, and I have to push forward to release it. (the machine is stone cold when this occurs) I've had the rear brake out twice now to clean and inspect it, and all appears fine. I suspect something is amiss with the auto adjusting mechanism, but I can't see it.
All this said, boiling fluid typically takes many minutes to cool enough to return to normal function especially if the car is stationary, so if the pedal goes to the floor then almost instantly fixes itself (like after an engine restart as described above) then that points more towards an internal issue in one of the master cylinders. Could be caused by several things like a deteriorating seal, a sticking piston cocking a bit and distorting a seal, congealed brake fluid distorting a seal, fluid contamination, etc.
I would start with a full and proper flush/replacement of the brake fluid and see what that yields in terms of information and results.
Last edited by Bitsobrits; 25/03/21 03:58 PM.
