What you will need

Austin/Morris Mini brake limiting valve. Metal or plastic machine knob with either 10mm or  3/8" thread. Matching jam nut. Small washer. 3/8" UNF bleed screw.

The main component of the bias valve is the cast brass pressure-limiting valve from a 1961-1976 Austin/Morris/Leyland Mini. Due to the Mini's popularity, these items are still available new. An even cheaper solution is to pull one off an old Mini at the local wrecker's yard. You'll find it bolted to the front section of the rear sub-frame.
The original purpose of this gadget is to limit the pressure in the rear brake lines to prevent the Mini's rear wheels from locking up under heavy braking.
There are three line orifices in the valve body; the front one is the inlet from the master cylinder, and the two side ones are the outlets to the rear brake drums.
Note: The flare on the inlet line is a single (convex) flare, while the outlets are double (concave) flares.
Upon closer inspection, you'll notice a small vent in the end, opposite the inlet. This hole needs to be drilled out, and tapped to suit the thread of the machine knob you have.

Carefully dismantle the valve, noting the sequence of the parts as they come out. If you've got a second hand valve, clean it thoroughly before proceeding.
Remove all traces of swarf and oil with an airline and degreaser. If it's a second hand valve, you would be advised to replace the seals. Lubricate the internals with the same type of brake fluid you'll be using in the rest of the system, then drop the small washer into the valve, followed by the spring and plunger. Screw on the end cap, and tighten it. Insert the bleed screw into one of the side orifices. Screw the machine knob into the newly tapped hole in the end, with or without the jam nut, depending on whether you want to adjust the brake bias on the fly, or have the bias fixed.
That's the conversion finished!

Mount the bias valve in a handy location, or, so the knob protrudes through the dash panel. Connect the inlet line to the valve using a single flare, and connect the outlet line to the free orifice in the side, using a double flare. Screw the bleed nipple into the remaining orifice in the side of the body. Bleed the brakes as normal, not forgetting the new bias valve too.