WTB 73 + disc/drum proportioning valve

At the risk of starting an argument: The part folks believe is a proportioning valve is little more than a switch housing for the brake failure warning light. It has no proportioning properties to it at all. It is a glorified "TEE" with a shuttle in it for tripping the light on when one end, front or rear, is getting less pressure than the other. Other wise it is just sending the brakes their respective fluids front to rear. The part shown is the same as used in the drum brake system. It is a common misconception. If in doubt take one apart and measure it out its internals and you'll see.
The proportioning needs to be done in line via an actual proportioning valve that's available from virtually everyone in the after market industry. Even MP had one for a number of years and, I believe it, was actually made by Willwood. The key is the residual pressure valving; also in line and made in the aftermarket. I think it is 5psi for discs and 2 for drums but don't take my word for the pressures without checking with a MFG as to their system requirements.