[scoobeesnac]

Hi all,

We had a brake fluid service on our F31 328i at BMW about a few months ago and it felt spongy so they did it again and since then I felt that the brakes were not releasing properly.

Today while driving at 45mph the brakes applied without my intervention and the pedal was rock hard to press.

I managed to pull over and the car seemed to sort itself out but I'm concerned of course how safe it is to drive.

Would appreciate any thoughts/suggestions what it could be?

I've read about failure of cylinder but in those cases I would expect to lose brake power rather than it automatically braking.

[Londoner]

No idea mate, but that doesn't sound like a safe vehicle to drive - book into dealership/garage tomorrow?!

How harsh was the auto braking?

[scoobeesnac]

Originally Posted by Londoner

No idea mate, but that doesn't sound like a safe vehicle to drive - book into dealership/garage tomorrow?!

How harsh was the auto braking?

Hi, braking was rather progressive, I don't know enough about the system to figure out how that's possible!

I'll take it to a garage tomorrow for a check for sure, I also need to do more research to see if anyone else has experienced this.

Cheers

[frazervalley]

Do you have cruise control with auto braking? I experienced something similar when I descended a steep hill on a highway while the cruise control was active. I was surprised to feel the car auto brake so as not to exceed the speed set on the cruise control. Checked my manual when I got home and realized that I had cruise control with auto braking.

[scoobeesnac]

Originally Posted by frazervalley

Do you have cruise control with auto braking? I experienced something similar when I descended a steep hill on a highway while the cruise control was active. I was surprised to feel the car auto brake so as not to exceed the speed set on the cruise control. Checked my manual when I got home and realized that I had cruise control with auto braking.

I do have cc with braking (not adaptive cruise though) but in this situation the car was bringing me to a halt for whatever reason.

Maybe handbrake or cylinder failure - I did wonder if the heat/dry cylinder pistons may be at play- but I don't see how the brakes can get applied like that :-/

[Alscoob]

Maybe brake servo fault.

If they leak internally they will pre load the master cylinder and effectively apply the brakes gradually. Ultimately the pedal will be solid. If you leave the car long enough the pressure will disperse and off you go again. If it is this it will get worse, so get it to a garage PDK.

[scoobeesnac]

Originally Posted by Alscoob

Maybe brake servo fault.

If they leak internally they will pre load the master cylinder and effectively apply the brakes gradually. Ultimately the pedal will be solid. If you leave the car long enough the pressure will disperse and off you go again. If it is this it will get worse, so get it to a garage PDK.

Thanks for the insight, I'll go get it checked out.

Cheers

[Eddamoo]

After you feel it braking, put your hand near all 4 disks to see if one is warmer than the others. It doesn't sound like a stick caliper but worth a check.

A stuck caliper can heat the fluid and cause a hard pedal.