1987 Mazda Truck Pedal dropping

Someone else wrote in with the same Question, but I have done all that you suggested and still have the same prob. I have replaced all brake lines, shoes, pads, master cylinder. I performed the test the book mentions and still the pedal will travel almost to the floor on the first step I still have brakes at the floor but doesn't seem right. comes up on second step and I did get some more pedal after adjusting the rear shoes. could it be the metering valve on the firewall, or is this common for this model?
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