Last week all was well with the M5, other than low pads on the rear.
Saturday I replaced the rear pads/rotors/pad wear sensor with new OE BMW. Took the car out and bedded the brakes, all was well.
Sunday I drove the car to my parents house for dinner. ~5 minutes into the drive the car felt down on power. I assumed a sensor was on it's way out, and it would throw a light soon. Proceeded. ~5 min after that a slight shaking occurred. I thought maybe I hadn't correctly torqued the wheels, pulled over to check (used a torque wrench, but better safe than sorry).
Both of the rear brakes were smoking when I pulled the car over, and the center caps had fallen out (assuming they got soft). I drove the final 5 min to my parents house, rear brakes dragging all the way, and then back home again. Good thing I bought the pads/rotors from FCP, so I can warranty swap them after this issue is sorted, as they are pad deposited for sure at this point.
Monday we didn't drive the car, because of the brake situation. Monday night I went to look at it, and the calipers had released. Did a 4 mile test drive, all was well (outside of the pad deposits).
Tuesday I sent my wife to work in it. Tuesday at lunch I get a call that the brakes were dragging again. I told her to limp it home.
Wednesday night I look at it. Calipers have released again. But... at this point I'm not fooled.
That said, I am at a loss as to what could be causing this situation. Anyone ever had an issue like this before? What was the cause?
It's both of the rear brakes, which makes me think it's not a line issue and unlikely to be a caliper issue. It's not the front brakes, making me think it's not a master cylinder issue. I can see pad deposits forming on the rotor, so I'm reasonably confident it's a pad grabbing it, and not the brake drum. Maybe a DSC hydro unit issue? But, if so, seems weird that it started coinciding with a brake pad/rotor swap-- feels like that should be involved somehow.
Probably not related, but in the interest of giving all possible related evidence: While I was doing the brake swap I also added wheel studs (TMS) and adjusted the ebrake (I could still turn the rotor freely by hand with no noticeable drag).
Please help! TIA.