greetings from snowy northern arizona! 2008 535xiT with a rear braking issue i’m struggling to diagnose. this issue presented last weekend.

when initially driving, there is no issue with the brakes. however, as pressure is applied to the brake pedal, the rear brakes are staying engaged and not releasing pressure. this only happens to the rear brakes. when allowed to sit over a 30-min period, the pressure to the rear brakes dissipates and returns to normal/neutral function until the brake pedal is reengaged. i don’t know if there’s a proportioning valve somewhere that isn’t allowing pressure to return to the master? or is it the master itself? or ABS related?

this issue presented last weekend while driving on very snowy/icy and uneven roads in northern arizona where we’ve had a huge snow year. at the time i noticed the wheels were spinning and there was some scraping over icy chunks in the road. i initially thought it may be the emergency/parking brake and something had been stuck against the cable. however, after inspecting that system i found it to be operational.

any advice is appreciated.

 

I'd suspect the "normal culprits" - a worn out caliper (or two), and/or sticky / rusty / dirty guide pins the calipers ride on.

It would be a good idea to get the back of the car in the air to test how much drag is on the brakes before and after application (to see if you can replicate the problem in a controlled environment).

It might also be interesting to shoot the temperature of the rear rotors after driving around for a while, to see if they're dragging (if they are, they should be getting pretty warm, even in northern Arizona in March! ;-)

 

thanks! it would surprise me if both calipers failed simultaneously, but strange things happen. after a short drive around the block both rear rotors are HOT while the fronts are cool. i have put the rear up on jack stands and and can confirm that the brakes will lock and stay locked when pedal depressed/released. after 30’ish minutes the pressure has dissipated and the wheels will spin freely.

 

Sounds like stuck guide pins to me. When was the last time they were cleaned. When was the last fluid flush?

 

Thanks, Sean. On any other (read "normal") car, I'd suggest the same (or possibly rusty / bad calipers - relating to your fluid flush question). But many times when I dispense "normal advice" someone will remind me "no, no, BMW doesn't do it that way". Thanks for confirming that - in this very limited way - a BMW can be "normal"... ;-)