Likely one of 2 problems. Either the pistons in that caliper are seized or if the brake lines are very old and rubber may have swollen up inside to the point where they're putting fluid pressure on the caliper pistons, which us like applying the brakes. Thus is one reason why the braided stainless lines with Teflon cores are better - they don't swell internally.

Also, when was the last time you changed your brake fluid? Brake fluid is hygroscopic which means it absorbs water. Nif it gas been years, that water in the system willninternalky rust the brake components in the master cylinder and especially the caliper pistons.

Most likely you're going to need to remove the stuck caliper and do a rebuild on it by removing the pistons, cleaning those up and replacing the seals.