virginian handyman

the shower next to my toilet appears to be hogging the pressure from the toilet inlet water line at certain pressure. Once the shower takes over, it robs the cold water from the toilet inlet leaving my tank empty. I have replaced all valves, cold water inlet line, the entire control flushing system in the toilet a few times. Nothing works. I even cut the drywall behind the toilet and re soldered a new t-joint split between the sink, toilet and shower. But the cold water line still fails to fill the toilet tank at certain pressure what appear to be dropping once the shower is on. This may be rare, but I suspect that the pressure balancing valve in the shower has gone bad and it takes total control over the cold water each time extreme pressure activates it, restricting the flow of inlet cold water linen to the toilet. My puzzle remains if that happens, why does the cold water in the sink still running fine and why does it only affect the toilet tank? Any advise to my diagnose is gratefully appreciated. here are my questions; 1) does pressure valve go bad and act in total control of water line? 2) If that happens, why would it only affect the toilet, especially since all three items are tied together? thanks
please email me your responses to wong.handyman@gmail.com if all possible.