...If the shuttle is indeed moving and blocking the rear outlet,...
Ken H.
That would prevent one from bleeding the rear brakes. As soon as one opens a bleeder, the pressure would drop and the shuttle would block it off.
When not bleeding, with the gauge attached, if it doesn't block it completely, the pressure would rise until it matches the pressure at the master cylinder.
It either blocks it completely and you can't bleed the brakes, or not completely and the pressure will equalize.