Front left brakes won't bleed, but will bleed while truck is running. All other calipers bleed fine while truck is off.

What is your question?

In process of flushing all brake fluid. Currently trying to flush last caliper (front left under resevoir) but no brake fluid is coming out. If I turn the truck on, brake fluid comes out like normal. Front left brakes seemed to be dragging so I replaced the caliper, but issue still present.

What makes this problem better or worse?

Not sure, but I think it gets worse after being allowed to warm up.

How long has your 2013 Ford F-150 had this problem?

A few months