"A" in the photo IS the factory, fixed proportioning valve.
This is what you would replace with the 'tee' fitting.
"B" is the shuttle valve. That valve is a safety feature. It has six ports. Front brake line enters/exits on one side (2 ports), rear line enters/exits on the other side (2 more ports). Your brake light switch is the switch mounted in the forward port on the driver's side of the shuttle valve (port #5).
In the middle section of the valve is a sliding piston in a chamber that connects the two sides (L & R). That piston has a beveled detent (picture an hour glass) in the middle and can move to either the left or the right when pressure is applied. In the photo, under the booster in the middle position of the valve to the rear is the switch that controls the brake warning light on the dashboard (port #6). That light is also controlled by the wear down wiring system on the brake pads, but this feature is disconnected on probably all remaining panteras.
That switch has a plunger that rests in the detent of the sliding piston. As long as there are no breaks anywhere in the brake tubing, the pressure on both sides of the connecting chamber is the same (more or less) and thus the piston will not move. Should you break a line somewhere that tubing leg (F or R) will not be able to build up pressure - you'll just be losing fluid through the break. When that happens the good side now has more pressure and that side's pressure will move the shuttle valve piston to the other side of the chamber. It will move the piston enough to fully - or mostly - block the passage for the bad leg of the tubing. Hopefully retaining enough function to at least leave you with some brakes. Also when the piston moves, the detent in the piston moves and the warning switch plunger is moved (depressed into the switch) and completes the circuit and the dash warning light comes on.
Still with me?
Sometimes bleeding the brakes will result in that shuttle valve piston moving. And sometimes, since it hasn't moved in years, if ever, there may be enough varnish built up in the outer edges of the piston's chamber that once it moves, it is stuck in that varnish. If you are lucky, like I was a couple of years back, closing everything up and working the brake peddle will force the piston back to the central postion and all is good. If it is truly stuck, you now have the joyful task of freeing the piston in the central chamber. Figure out which end of the car is getting fluid and you'll know which side is now blocked by the sticky piston. Removing a fitting and using compressed air may push things back where they belong. If not, well, remove the tubes, remove the valve and clean the varnish with your favorite solvent.
You can prevent the piston from moving during bleeding by removing the switch and threading in a dummy stop that will index with the piston's detent. Some folks do this permanently, but that voids the safety feature the shuttle valve provides.
If you are adding a proportioning valve, you would want to interrupt the rear brake tubing after it leaves the shuttle valve.
Remember, ideal brake function will always have the fronts locking first.
Sorry for such a long post. Confusing subject, and even more so if not clearly explained - which I hope I managed to do.
Larry