Here's a pic of the four wheel drum distribution block that I am using for my four wheel disc conversion. The car originally had front disc rear drum AND a factory proportioning valve, which had to be removed. Properly designed and balanced brake systems that have either four wheel drums or four wheel discs use distribution blocks.

The "proportioning" of the brake bias from front to rear is governed in general by the size of the caliper bores in relation to each other. Rotor and pad size and composision also plays a role.

A proportioning valve is typically used in an attempt to balance the proportion of front to rear brake power. In general, it is a bandaid for a less than ideal brake design such as matching front discs to rear drums. In certain cases it can be a useful tool to balance the proportioning of brake bias during races to compensate for changing fuel loads, track conditions, tire wear etc.