1980's brake system disc/drum

The aluminum master cylinder from the late 70's onward is used on lots of older Mopars.

In looking at the factory manual (1983 Chrysler), the brake system (disc/drum) uses a single proportioning valve/distribution block/hold-off valve assembly as pictured.

So here's my question....where is the residual pressure valve? Since the drum brakes in the rear need to have some pressure held in the line, traditionally the master cylinder has the residual valve (RV) behind the brass tube seat. But, the famous aluminum MC does not have an RV. Neither does the prop valve assembly. So how is this function being accomplished?

Aluminum MC.jpg

Prop valve.jpg