1) A valve can regulate or restrict flow of a fluid. A switch is for cutting off the flow of electricity, not a fluid. Calling a Yugo gas valve a switch is asinine.
2) I have owned 3 different yugo SKSs with gas systems so corroded they wouldn't function as semiautos and I've heard of countless others with the same problems. All of them worked great as soon as the gas piston and/or gas tube were replaced.