The 'combination valve' I used in my system has a proportioner function that reduces the pressure to the rear brakes during maximum braking. The metering function delays the front pressure for a bit to allow the rear shoes to make contact before allowing pressure to the front discs. The proportioner function only reduces the pressure to the rear during a panic stop, otherwise both front and rear get the same pressure. Photo is a paragraph from the GM shop manual for the vehicle that my combination valve came from.
