Now that I have your attention, I have to mention that this is for my Ford not my 'velle. Anyway, I have a 1979 service model LTD Crown Victoria. I put a rear out of a '93 Merc with disks in it. I used a dual diagrapham booster and aluminum master cylinder from the Mercury also. The proportioning valve I used is for a disk/disk firebird. I am having some trouble getting the rear brakes to function properly (all brand new). I am not sure if I am supposed to run the rear line through a proportioning valve or not. There is a valve on the side of the master cyliner where the rear line comes out. What function is performed inside of the proportioning valve of the F-body as far as the rear brakes are concerned?

It reduces the rear brake pressure in proportion to the front during hard stops. The valve inside the 93 Mercury master likely does the same thing. Don't use both.

If the Firebird valve is cast iron, you may remove the valve from inside of it. If the 93 Merc master has a service plug for the valve (probably opposite the port on that model), it can be removed.

It's your decision which one was designed to reduce the proper amount of pressure for your vehicle's needs. I'd scrap them both and use an adjustable.

The proportioning valve is not iorn, it is brass. The master cylinder does have a bleeder valve on the opposite side of the port with the valve. The valve unscrews from the side of the master cylinder heaving a large hole to be filled and reduced to flare fitting size. What should I do to remove the valve from the master cylinder?

If it does not have a service plug it probably does not have a proportioning valve in the master cylinder. If in that hole you find a piston, and behind that a spring, they are the proportioning valve. I am not familiar with that model, but they usually fall right out, or can be drawn out easily, just hanging on an oring. It's possible for there to be a snap ring.

The brass GM valves have integral proportioning inside the rear nut, not serviceable.

Thanks for the help. I will take the master cylinder apart tomorrow and check it out.