I've opened up the reservoir and tried resetting the piston on the bleed nipple side and it won't budge. I haven't ridden this bike for about a year and the front wheel was off, and I also noticed when I was getting it out that the bleed nipple cover had come off/been taken off of the bleed nipple (if that's any contributing factor to the compressed piston).
Does anyone know what I need to do about this? I've tried a few times to open up the reservoir and them push the piston back with quite a lot of force and no results. It's pretty far out too, the pad sits a few millimetres off of the side where the pin is located, and the tire can't spin at all and doesn't sit completely in between the pads (they end up at a V shape because the pad is being pushed out by the piston but is still magnetised to it).
It's also just the one piston, the other is perfectly in place.
Also, I just pinched this picture off of their website: https://i.imgur.com/ZYV5YWC.png Just to reaffirm, the picture isn't my brake, but the brake pad indicated to by the red arrow is the brake pad that is not sitting properly and it being pushed out by a compressed piston.