Front disk brake(s) sticking- where to start toubleshooting?

My project has been subject to long starts and stops. A couple years back, I replaced the calipers, rotors, wheel bearings and flexible lines on my Dart's front brakes. I used OEM replacement parts.

There's honestly not a lot that can go wrong with this job...but I didn't get around to bleeding the brakes until very recently. They sat for a long time.

Now that the car is back on the road, I find that the pedal will feel normal (pretty damn good, actually), then the brake doesn't release entirely, and the brake predictably gets super hot. Fortunately, I haven't driven too far!

The change is detectable through the pedal...it loses travel and feels harder. Usually, it doesn't happen right way...but when I jacked up the car yesterday, the wheel was clearly being held by the brake, even when cold.

First thought is that the pistons corroded and aren't sliding back into the caliper. What would this look like? Could I see this without taking the caliper off?

Is there any way to fix this (lubricant?) without replacing the calipers?