Brake calipers are actually some of the simplest and most straightforward repairs/rebuilds you can do on a car.
It's just a lump of cast iron or aluminium, with 1 moving part (piston) which is connected to the bracket wth 2 sliding pins so the caliper can move.
If it's seized it's because one of those 2 things (piston or sliding pins) are not moving freely. If the parking brake is cable operated sometimes the problem could be a frayed cable, but sems not to be the case here.
Very easy to remove the caliper to check the pins. Remove wheel, disconnect hoses/cables from whatever fixing brackets there are (just to get more movement. No need to disconnect from caliper), undo brake fluid reservoir cap (don't leave it open too long. It absorbs moisture from the air and gets f*cked), use a screwdriver between pads and disc to compress piston and get some clearance, close reservoir cap.
Now just remove two rubber caps at the back of the caliper, that give you access to the pins. The pins come out with an allen key through the hole the caps were protecting. Unscrew the pins out, and the caliper can come out with a bit of wiggling.
Now just reinsert the pins in the caliper and check if they move freely by hand. They go into rubber bushings which chances are will be dried and perished preventing free movement. (Replace rubber bushings, clean pins, and reinsert them with a load of lube. Job done)
Check the rubber boot on the piston. If it's cracked, ripped or perished chances are the piston is full of crap inside and rusty, preventing free movement.
Removing/replacing the piston is a little more involved but it only takes 5 minutes. Plenty videos around or we can run you through it if you want to go that way. Only thing with that is the brake fluid will need filling, bleeding and flushing, which is not a bad thing and should be done every 2 years at most anyway. (Absorbs moisture and gest f*cked, etc) but needs a bleeder ideally (cheap).
Rebuild kits can be bought everywhere and are cheap.They can be a basic kit of just gaskets, sliding pins rubber bushings, red grease to lubricate them, caps, etc. Or you can get them with new pistons as well. These guys are the go to for kits and they also have an ebay shop
https://www.biggred.co.uk/What i do with all my cars, i buy used calipers on ebay for peanuts. Rebuild them, and then do a straight swap. And so other than not having the car unavailable while rebuilding (i take the chance to paint them) i also have a second set ready at all times.
Some cars need a piston winding tool for the rears. If you want, i got one one i've been trying to get rid of for years that you can have
