Loosen the steel line at the master cylinder to see if the brake releases. If it does, suspect the brake fluid is contaminated with a petroleum product. You'll find proof of that by inspecting the rubber bladder seal under the reservoir cap. If it's ballooned up and mushy, the same thing has happened to the rubber lip seals inside the master cylinder. They've grown past the fluid return ports and blocked them off.
If the brake doesn't release at the master cylinder, loosen the bleeder screw on that caliper. If that allows the caliper to release, suspect the rubber hose is coming apart inside and acting like a check valve. That's not real common on your car but it is common on some other brands and models.
If opening the bleeder screw doesn't allow the caliper to release, a ring of rust or dirt has built up around the piston and the caliper must be replaced or rebuilt. We used to always rebuild calipers many years ago during routine brake jobs but today it's less expensive to just buy rebuilt calipers. For even braking, both front ones should be replaced at the same time.