J, you may already know this, but if you're doing wholesale brake changes to your car, you can entirely delete the shuttle valve under the brake booster. The moving shuttle inside the valve has no spring returns, and varnish tends to build up at the ends of the shuttle passages. Then if for any reason- including aggressive brake bleeding, the valve travels to the extreme end of its travel, it tends to stick there.
That essentially cuts off almost all fluid flow (and stopping power) from either front or rear brakes, depending on which direction the shuttle moved. Kind of a bear to diagnose if you're unfamiliar with Pantera brakes. It's also a messy job to move the shuttle back to its center position without removing the assembly from the front trunk. Its only function is to turn on a red warning light in the dash, ostensibly for low brake fluid warning, but two other things also turn on the same light, diluting its function.
Note also the stock (Ford?) plumbing for the stock non-adjustable proportioning valve limits brake pressure to the entirely adequate FRONT calipers while running full pressure to the toy-like rear calipers, for reasons that have always escaped me. If you install a manual proportioning valve for brake balancing, remove the stock one as they will fight each other.