Is the piston siezed, or is it the slider bracket?
If the piston is siezed, you can do what we used to do at SSBC, air compressor to blow it out. BUT, THIS IS DANGEROUS for obvious reasons. Make sure to put a woden block in between the caliper / piston so it doesn’t come shooting out.
If the slider pins are siezed up, well, its a real bitch to get out. You just have to keep working it with plenty of penetrating oil and use vise grips to work it out.