#1 culprit - Rubber brake hose collapsing internally. You won't be able to see the collapse by looking at the hose. It's a common problem on all hydraulic brake systems.
Plugged return port in the bottom of the master cyl. can do the same. With the cap removed, very carefully squeeze the lever and see if fluid squirts up out of the reservoir. Use rags so fluid doesn't end up on your paint.
A stuck caliper wouldn't release by opening the bleeder, so it's not that.