Proportioning valve

[myt-yt-67]

:confused: I have a 67 camaro that came originally with power drum brakes all around. I put discs on the front from a buick skylark. I put a new master cylinder and proportioning valve at that time. The brakes work fine. I have now put after market discs on the back and the brakes are not good. Do I need to replace the proportioning valve and the mater cylinder or just the proportioning valve alone? Or is it something else.
Any help would be greatly appreciated. Thanks

[CPPAaron]

Typically its a prop valve only; all depends though on were the unit came from. CPP units dont even require changing the valve! See this link to change a 2 wheel type to a 4 wheel unit!
Classic Performance Products (http://www.classicperform.com/Instructions/PDF/Prop_Valve_Mod_Instructions.htm)