I'm a bit late to this thread - glad the OP (Dan) got this sorted. I would be really interested in the dissection verdict if Larry ever manages to get hold of the parts that caused the problem.
Can't be the SMC as I only activated the brake pedal.
That is not a correct assumption. When the brake pedal is pressed, it activates the rear centre piston via one line. However, via another line, it also pumps fluid to the centre piston of the front callipers, and the same line also feeds fluid into the SMC, behind the main 'plunger' (the primary seal). When the bike is stationary, this pressure pushes fluid past the primary seal and activates the two outer pistons of the rear calliper - just as if the SMC had been activated. If there is an issue with the SMC not returning properly, this pressure from the SMC cannot be released. One trick is to find out which piston is clamping the pads - put a strip of paper between the back of the pad and each piston to see which one is still pressed firmly against the disk pads. Don't forget to remove the paper afterwards !
So, I removed the pad closest to the piston - fairly easy but the other pad still drags a lot. My conclusion is that the sliding pins aren't working properly. What else could it be? I cleaned both of them and added some silicone grease. I am not super impressed how the bracket slides...So maybe I need a new bracket (I replaced one pin plus the rubber boot). Could it be that the hole in the bracket has corroded beyond repair?
This is revealing. That hole that the pin slides in is a good bit larger than the pin. There are two pins and two holes. One pin is on the calliper, one pin is on the bracket. It is physically possible to appear to have the calliper on the bracket when only one of the pins is properly mated. At least, I managed to do it once. If the hole has corroded, that would be a heck of a lot of corrosion. Get a drill bit of the appropriate size and turn it by hand to ream it out. It could be muck in there. Or its possible that the pin is bent/cross threaded.
Its also possible that you/someone has used too much grease - recently or in the past. Old grease in the hole will stop the pin from sliding to the bottom. New grease could create an air lock and air pressure keeps the pads against the disc rotor.
Also possible is that roof spring in the calliper. They have an odd design which allows them to be fitted the wrong way round. Take a look at it. The narrow spring strip which rests on top of the pads should be on the inner side of the calliper - ie resting on top of the pads which are closest to the centre of the wheel and furthest away from the pistons. This is crucial. If the narrow strip is at the other side, than as the pads wear down, the pistons move out and the pad gets pushed away from the spring and this can then interefere with the braking.
Also on the narrow strip are two tiny tabs which protrude. The innermost pad is supposed to sit behind these tabs so that the pad cannot move across to the disc rotor. If care isn't taken when inserting the pads, they can be bent over - again interfering with the brakes.
Take the other set of pads out. Take the spring out (and before you lose it, remove the clip at the far end of the calliper - where the brake pad tab sits). That rear clip should have a ridge on it. If you have pads which do not have a notch at the tab end, then the pads and calliper will not be bale to move across. With no brake pads and no springs, that calliper should move across easily if both pins are mated in their holes. If not, check the holes are clear. Then check that the pins are in line. If all is OK, then you need to look seriously at the mounting of the pins on the calliper and on the bracket - and check for cracks.
Take a look at my
'ST1300 Brakes -Avoiding the Pitfalls'. Its not definitive, but it highlights most of the problems with brakes and it has some pictures to illustrate much of the above.