1966 valiant dual master distribution block

The unit pictured by Pista integrates a prop valve in the rear line path; the black thing on the lower right end of the pictured unit is the cap on the rear prop valve. The top plastic covered thing is an electrical connection for a fail switch, that activates if either front or rear systems fails and loses pressure.

OP, is your setup a drum-drum or disc-drum? With a drum-drum, a prop valve is not used. The sizing of the drums, shoes, and wheel cylinders sets the front-rear proportioning.