FWIW...when I redid the brakes on my 68, I eliminated this distribution block/safety switch. I was converting the rear to BBP with B body 10 x 2-1/2" drums. The fronts are BBP 73-76 discs. Since I had it all apart anyway, I plumbed in a Line-loc for the future 4 speed and added a cheap Summiit (Wilwood) adjustable proportioning valve.
The front brake line goes from the MC into the Line-loc then into a simple tee with outlets to each front brake. The rear line goes from the MC into the adj prop valve then rearwards to the rear brakes. I no longer have a safety switch which does nothing more than turn on the dash light if it senses a leak in the system. I had to custom bend several lines and ended up having a small leak. How did I know w/o the safety warning light? I stood on the brake pedal and held it. Gradually the pedal started moving toward the floor which told me I had a leak. I found the leak and "fixed" it by turning the fitting tighter. So now, I test it occasionally by doing the same simple test at traffic lights. This may not be a good approach for everyone but I like it simple and the fewer connections, the better.
The picture shows the alum MC and you can just see part of the adj prop valve mounted to an aluminum bracket underneath the MC and the Line-loc on the inner fender. Those are temporary bleeder lines going into the reservoir cause I bled it on the car, not the bench. Excuse the messy wiring, it needs to be re-done.
