Proportioning valves limit the rate of application, not the pressures. The rear brakes apply slightly ahead of the front to avoid swapping ends in slippery conditions. Does that training car have pressure gauges on both brake circuits? Somebody please measure those MC pistons and post the diameter differences. Jerry
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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.