An air bubble would not give this symptom. It would give a softer spongy pedal feel.

When you have a caliper seize it is usually the piston not retracting because of contamination. Get a new caliper. Flush the old fluid and put in all new fluid. Would be a good time to put on sone new brake lines too.

I've replaced two calipers after they seized. Thought about rebuilding them but do not have the time.