The first thing I would check is the condition of the brake pads. If they are
severly worn, the pistons that push the pads outward may be sticking out so far
that they are binding in the caliper housing.

Second, I would look for corrosion of the caliper and pistons -also allowing
them to bind or seize.

Third, I would look at the front brake master cylinder - it has a valve
internally that allows for fluid to return from the caliper when the lever is
released.

Lastly, I would replace the flexible brake hose connected to the caliper. This
is least likely, but I have had an internal failure of the rubber hose on an RV
behave exactly as you are describing. The hose is usually a "hose within a hose"
and the inside liner fails and blocks return fluid.

The order I have listed is, what I believe, the most likely to produce the
symptoms you have described.