Hi, first if you are unsure how to sort out the brake caliper / pistons / rear brake cylinders DO NOT TOUCH THEM, sorry for emphasizing, the piston within the brake caliper at times gets rust on the piston this in-turn rubs & sticks within the caliper walls/ casing if that's the case 7 whoever has tried to clean out the rust then a replacement is needed, you can either get a 2nd hand one but myself I would go for a replacement, that means exchanging your for the recon part as some manufacturers do, the rear brake mechanism, the rear is a different system as it also contains the handbrake / cables that are attached to the brake linings, there may be dislodged brake lining spring that may have gor to the side of the lining within the drum this could cause the sticking brake you can only find this out if you remove the road wheel & the drum, but again if you are not aware of what to do, DO NOT DO IT, you said a CLUNK!! get the drive shafts checked, what ever avenue you go down If unsure LEAVE IT and get someone who knows to sort it, too many have tried to sort out these problems & come unstuck.