The pin is only going to move about 1/32" to 1/16", just enough to open the valve and allow fluid & pressure to enter the front lines.
You don't have any crimped lines up there do you?

Rears locking up on the first run on a damp day isn't really all that unusual, there's no weight back there.

Sounds like the shoes in your rear drums are not fully retracting. Assuming you've driven around for an hour or so... If you put the truck in gear, does it roll along okay at idle on a flat level surface, or does it feel like your dragging an anchor that wasn't there before... or does it perhaps have an intermitent drag, like someone is tugging on the rear every few feet? Are the rear brakes getting abnormally hot?