You don't "need" a proporsioning valve
WRONG! VERY WRONG! Even the spelling.
Disc brake calipers require more pressure than drum brakes. That means the rear brakes will lock up before you get any braking action and the truck will swap ends.
In that ecihotrodbrakes, drawing, I think what they meant was that an "adjustable" valve was optional.
They also drew the metering valve as though it were a "T" fitting. On most disc brake setups the pads are already against the rotor. The rear shoes are not against the drums. The metering valve holds off the discs until the drums can contact at the same time. The proportioning valve reduces the pressure to the rear so that front can do most of the vehicle braking.
Brake Valve Definitions Even this company says you don't need an "adjustable" valve. What they are saying is that a "combination" valve will give you metering and proportioning.
