Eric, the device you described is not the factory proportioning valve. It is the metering valve. This device is there to ensure that the rear brakes receive about 75 psi of pressure prior to the front brakes receiving their pressure. This is because it takes drum linings longer to contact the drum than it takes disc pistons to contact the rotor (because they have to overcome the return springs). The proportioning valve is the 'junction block' on the frame that you described.
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