1963 GMC K10 stock BRAKE question-problem wont stop

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1963 GMC K10 Brake question. All stock, no mods. I replaced every part except the metal lines. I don't or didn't see any reason to , was a very clean Montanna truck. This thing WILL NOT stop ! Rears will lock up if pressed very hard , fronts barely slow the truck down. All wheel cylinders brake hoses shoes are new, drums turned. I replaced it with three different, double, hydraulic clutch type master cylinders all NOS GM and one from a truck that works perfect , no change or difference. Pedal is at the top, no air in the lines and vacuum bled. I removed all the front metal lines after this, cleaned, ran a wire through them , cant find a restriction, rechecked wheel cylinders. The idea that the shoes needed to be arched to match the drum was also done. Still will not stop. All my other K10s stop fine and were easy. What am I missing ? Truck sat the last 10 years so the wheel cylinders froze , it all worked when parked but?