Front disc

Open the hydraulic circuit in different places as you work your way back up to the master cylinder. If the brake releases when you loosen the steel line right at the master cylinder, suspect brake fluid is contaminated with petroleum product. The proof will be the bladder seal under the reservoir cap will be ballooned up and mushy. The lip seals grow the same way and grow past the fluid return ports and block them.

If it doesn't release there, suspect the rubber flex hose. Some cars have a metal bracket crimped around the middle of that hose. Rust can build up inside that crimp and constrict the hose. The brake will apply under hard pedal pressure but the trapped fluid can't release. Simply opening that crimp up a little with a large pliers will solve that problem.