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I was returning from a track day where the brake got a lot harder use than normal , when I came up to some lights in town , I squeezed the brake and it felt like a pop and suddenly I didnt have any brake at all, I managed to stop ok and had a good look for fluid loss , couldn't see any . After pumping the lever with not much idea I got pressure again and the brake worked ok the rest of the way home .
Just lucky it didn't happen on track !
Any ideas ?.

I think I would at least be renewing the master Cylinder seals and the fluid - you may have cooked the fluid on track or it may be "water logged" if its not been changed for a while.

Old fluid, perhaps some moisture in it...hard use gets everything super hot, moisture boils off....air in system? Just a thought!

Pete

Blead brakes?!

Thanks guys, for replies .
Weird I've got full , solid front brake again ! without doing anything .
Cheers Gary.

which backs up what everyone advised, it's now cooled down.

There's good reason BMW recommend changing brake fluid regularly, which many fail to do, then find failed abs pumps as the moisture causes problems, or internal corrosion and overheat issues