I can’t get the brake piston to go back in
Try removing the brake fluid reservoir cap. The brake fluid is contained in a sealed system and sometimes it can be difficult to compress.Removing the reservoir cap will allow the fluid to move freely and make it easier to compress the brake caliper piston.
If that doesn’t help you can try disconnecting the brake caliper hose. The caliper hose over time may collapse and restrict the flow of the brake fluid not allowing it to flow back into the system. This would make it almost impossible to compress the caliper piston. If that does help, you need to replace the hose.
If removing the caliper hose doesn’t help you may crack the bleeder valve on the caliper and if that doesn’t help, you need to replace the caliper.