Another possibility is that the caliper and/or the master cylinder needs to be rebuilt. If the bike has to stay out in the weather, the caliper needs to be rebuilt about every 2 years. It only involves replacing a squared o-ring. Dirt and stuff build up in there just outside the seal and jam the piston from moving freely.
The master cylinder is a little harder. It pushed brake fluid down the line(obviously). When the brake handle is released, a spring pulls the pump piston back, while a check valve opens allowing the fluid to come back into the fluid reservoir. The check valve often fails, not allowing the brake to release. So whether the caliper piston jams, or the master cylinder check valve fails, the same symptom appears. Rebuilding the master cylinder involves long, needle-nose snap-ring pliers. You should also flush out all the old brake fluid(don't be tempted to use DOT 5). If you add replacing the pads, you've completely rebuilt the front brake.