1st Gen R50 Cooper Most liked posts in thread: Handbrake cable, or seized caliper?

Those cables frequently rust over time. If you disconnect each cable at the caliper end, you should be able to push and pull it through the housing easily with your fingers. If it feels very rough or you can't move it with your fingers, it's probably rusted. New cables are the fix, if that's the case.

 

do the rotors get warm over time (even when not braking hard)? When calipers drag, things get quite hot - I had that happen with an older car and the entire rim was hot after a 20 mile drive. If there is no drag, they should be as cool or colder than the front wheels when you stop.

 

The thunk was probably the rotors releasing from the pads. You mentioned that it was raining: the rotors will corrode very quickly in wet conditions and stick to the pads until the car moves. You can check to see if the calipers or cables are faulty by putting the rear of the car in the air and removing the rear wheels. Take a pair of pliers and grab the ebrake levers (that have the cable ends in the bracket) and twisting them as if the cable were pulling them. If the levers move freely (they are sprung, so it'll take some effort) they're probably ok. Then have an assistant pull the ebrake and observe whether both cables pull the levers. If they both move freely, your cables are ok. If one cable is bad, replace both, or you'll end up doing the job twice.

 

all 4 of mine did it in southern california.