A couple of comments about the above from
@gunsmoker's friend - (not the toaster !)
First : When you fit pads, or refit the same pads the chances are they the wheel will spin freely. You have probably done this already, but you must make sure that the brakes are applied hard using the brake pedal AND by activating the SMC by hand so that the rear wheel is locked. And then check that the rear wheel can spin
after the SMC has been released. (Note that you need to use both - the SMC and the pedal. The pedal because that is the only one that operates the rear centre piston. The pedal will also apply the outer pistons, but it is still necessary to activate the SMC by hand so that you can check that the brakes are released after letting go. That is because it needs to move back into its correct position by itself and release the pressure.
If you do not, then you could have a solution that works in the workshop, but which fails on the road when the bike is moving.
Second: This is speculative, but may be very relevant. New pads often sssshhhhh when they are fitted, but they do not drag excessively. When I fit brand new pads they make a loud ssshhhing noise, and continue to do so as I back it down the drive. I don't hear it when riding, and there is no drag. But after a ride when I return it to the garage, that Sssssshhhhh noise has gone.
They occupy all of the space in the caliper with a tiny bit to spare. With new pads the pistons ahve to be pushed in al of the way (after cleaning them of course). If you find that the new pads are not fitting properly, that may be because the pads are not quite parallel to the brake disk, so there is no position where the new pads cannot be pressing on the disk. You cannot adjust this. But if it is the case, then there could be due to a number of things that cause the caliper to become slightly twisted. It is one of the things that can lead to the rear brakes locking.
Suspects are:
The slider pins - one on the bracket, one on the caliper - which should never be removed. But if it
has been removed and then replaced and torqued, the Honda manual diagram specifies a torque that is about 3 times what it should be. They have quoted the torque for the caliper bracket stopper bolt in error. If the 69Nm required for the caliper bracket stopper bolt is applied to the slider pin (which requires only 27Nm), then it will simply strip the threads. This tends to allow the pin to be screwed into the meass of the threads that remains, but often at a slight angle.
The caliper bracket stopper bolt. If it is not tightened first when refitting the wheel and caliper, there is nothing to keep the caliper bracket at the correct position when that massive toque is applied to the rear axle. If the stopper bolt is held loosely in place, the stopper bolt is free to wobble around on its threads. When the axle is tightened, the bracket will tend to rotate clockwise with the tightening of the axle bolt and press up agianst that free play and it will be at a slight angle. Which which will tilt the caliper bracket slightly. If the stopper bolt is tightened first, to its full 69Nm. That hefty bolt isn't going to move anywhere when the axle is tightened.
The recess into which the chrome pad retainer clip fits, could be badly worn, which sets the pads at an angle and can also cause the caliper bracket to twist each time the brakes are applied.
There could be an issue with the pad spring being fitted the wrong way round, or with the tiny retainer tags for the in board pad. Or you could be fitting Honda front pads to the rear caliper (check the notch in the tab end of the backing plate for rear pads). Front pads do not have the notch and they will bind against the ridge in the chrome retainer clip if they are used int he rear caliper.
Final thought. Check your brake disk. Is it running true, or has it got a light warp in it ? If there is a warp, then it will nudge the pistons back in a bit as you ride. You will know, because your brake pedal will feel 'long' on the first application - you have to press it a long way down before it starts to work, or maybe pump it a few times. After which it feels solid. The first pumps are pumping the pistons back out again. With new pads, there is no piston to push in - they are almost as far in as they get - so the brakes drag, and you will feel pulsing at the pedal. You won't feel pulsing with part worn pads, but you will ge the long pedal.
You have to be careful about finding a solution that hides the symptoms, rather than addressing the problem - like using the old pads when new ones do not fit. To me this is an early warning alarm. You can ignore it for a while, but it isn't going to get better. It can only get worse. Each application of the brakes presses the pads and the disk together - so at that point they have to be parallel - but that is only achieved by further twisting of the caliper bracket or the slider pins.
If you need further information about any of these things then just ask. The point is that if old pads fit 'OK' but new pads don't, then there is something like that going on and it needs investigating because it can only get worse with time.
I said - this is speculative. It is. But to me, what
@gunsmoker's friend describes is an early warning sign which deserves further investigation.