Hoping you guys can help out with this.

1988 GMC K1500 Vacuum Brakes.
Brake Pedal goes to the floor - very poor braking, to almost none.

Replaced Front Pads, Rotors, Calipers, Rear drums, shoes, cylinders.
Replaced Master cylinder

The only thing left is:
Vacuum booster, Proportioning Valve

Any quick tips on how to diagnose the Vacuum Booster or proportioning valve?

The prop. valve is pricy and hard to find as is the booster.

Any info is appreciated!