I know what they are. I stated that. It is not a hold off valve, it is proportioning valve. The KH system in 70 incorporated a hold off valve (or metering valve) for the front disc. Proportioning valve applies normal pressure to rear until a set point around 300-500 and which point it only pass half the pressure above set point valve. There is no delay or hold off with a proportioning valve.
I was confused by his statement of 2 proportioning valves.