Let's do some quick math. The volume of the pump chamber, multiplied by the number of strokes, equals a very small volume to return the lines to pressurized status. The quick bleed down corroborates this. What's the closest thing to the plunger pump? The Davco? You've changed the filter, and looked at the check ball, but have you installed a rebuild kit? New ball, spring, and seat ring. It doesn't take a very big spot on the o-ring to cause a complete failure.