For anyone that thinks they want two proportioning valves, they might consider looking at this pressure chart.
Proportion valve pressure reduction I'm sure you understand that the pressure from the master is always equal, for front and back. Two pistons the same size, moved the same distance at the same time.
The first thing you see on the chart is that below about 250 the pressures are the same. 250 to the front brakes, 250 into the proportion valve and 250 out of the proportion valve to the rear brakes. The rear of the first Camaro's were pretty light so heavier cars may have equal pressures(in and out of the valve) up to a higher level.
Looking at the chart at 1000psi, you see that after the prop valve(combo valve in ratrod's case) you get 600psi. If instead of sending the 600 to the rear brakes, you sent that to a second prop valve. Look at the chart. 600psi into a prop valve produces about 400psi. You now have 1000psi to the front brakes and only 400psi to the rears. Even if the second prop valve is adjustable, you probably can't adjust the pressure up to what you should have for the rear.
