New Pads + Rotors, mechanic says rear pistons are seized. Cyclical knocking sound while braking moderate to hard

Question

2012 Mini Cooper w/ 72,000km on it (about 40k miles). Just had mechanic put on new pads and rotors on all 4 corners. Old brakes were constantly rubbing and making noise. A different mechanic said my front pads were wearing out and needed service on caliper slide pins. They also said my rears were wearing very heavily on the inside pad, but not the outside.

Took the car to a different mechanic to do the work. He said the rear calipers were extremely hard to compress+spin inwards (he deals with a lot of bmw/mini so I'm sure he was using the right technique with the rotation).

After he took the car for a drive after the install, he put it back on the lift and was looking at the rear brakes. He came back to me and said he hears knocking and it is likely due to rear caliper piston seizing issue. While I buy that he says my rear calipers need replacement, I don't completely buy that a cyclical knocking sound can be caused by a seized caliper.

Can someone explain this?

Answer 1

After bringing it back to the mechanic, here's what was concluded: 1) Old rotors didn't cause knocking. 2) New rotors caused knocking. We suspect a defect in one of the rear rotors (Raybestos Coated) 3) Mechanic installed their own rotors, which solved the problem.

It looks like the I actually had a defective rotor causing this issue. But don't forget to check your lug nuts over as well!

We also found that there was more knocking from the front rotor, which we thought was another bad rotor. Turns out it was a front wheel that was left loose (lug nuts were on, but not torqued to spec).