I got the fright of my life yesterday! I reversed my 1986 series 3 off my driveway and touched the brake pedal which just sank straight to the floor and the car carried on backwards heading for a car coming down the street. I pressed the pedal again and this time they were fine. I drove it off the driveway with no more problems and going along my quiet street tested the brakes a dozen times and they worked fine. 200 metres later at a junction the same thing happened! It takes a few seconds for your brain to figure out what to do by which time I was half way out of the junction and heading for the back of stationary traffic!!!!!! Handbrake just slowed it a little.I figured turning off the ignition won't stop it and I would loose pas to steer around cars or into a lamppost to stop! I slammed it into reverse which locked the rear wheels then put it into neutral. When no traffic was near I reversed back onto my street and crawled home! I have checked underneath and no leaks from brake pipes, fluid level in reservoir is unchanged no warning light from pressure differential unit. I dont understand how this could happen, I thought there was some kind of mechanical failsafe. Anybody had this problem? Thanks. I'm just going out to fit a spare master cylinder I have in my garage.

 

No leaks, a cold start and movement off the driveway, and no brakes at all suggests it is the master cylinder. I have had no brakes syndrome, but only after over 10 miles of driving to heat up fluid due to a seized caliper piston. This was as alarming as your failure !!

 

Definitely sounds like the seals in the master cylinder. Often happens that if you apply the brakes gently the pedal goes all the way down whereas if you apply it quickly or pump the pedal then it doesn't, at least for a while.
Might also be a lot of air in the system.

 

I do a fairly rigorous brake test every time I leave the house before I get to the road. Hasn't failed me yet ,thank god.

Not really sure what I would do if my test failed. Been lucky the last 18 years with this driveway.

 

Hi, looks like u must live in San Francisco! It was the seals in the master cylinder. I fitted the spare me and it was fine so have ordered a repair kit as the m/c I fitted has done more miles and is approx. the same age as the one that failed