I sorted out a previous bike with the same seized rear piston and caliper slide.

To fix the piston you need to pump the pedal so the piston becomes exposed enough before coming right out (and making a right mess) then use fine grade sand paper to smooth out the ridges along the piston that would have built up. Working the piston in and out should free it up. I use a pair of pipe pliers to get a difficult piston back in (wider jaws).

The slides can be the same but use plenty of grease during reassembly. Could be caused by water ingress due to damaged rubber boots? A �5 fix.

Rear calipers get hammered as it's more difficult to access to strip and clean reguarly.