So I've had a problem with my Pulsar's (n15 sss) rear left brake for a few months now. The problem is that there is a squeaking/rubbing sound whenever the car moves over 5km/h-ish. I've worked out that the reason was the piston in the caliper would push the brake pad onto the rotor when you pushed down on the brake but then wouldn't fully-retract when you let off the brake, which leaves the pad slightly rubbing the rotor.

After figuring out the problem I pulled the pads out of the caliper and grinded them down about 1-2mm to see if that would result in a quick-fix, which didn't work unfortunately. I've also bought a rear-left brake caliper off another n15 sss from a local wrecker's, but after replacing the new caliper with the old, the problem is still the same.

Would anybody have any ideas or have had a problem similar to this? Cheers

Notes: - Car sat for like 5-6 months without being started before I grabbed a new battery and started it up.
- Rotors had a layer of rust over them after sitting for awhile (other brakes were fine after a 30 minute drive).