Brake troubleshooting

[Jayred]

I have a braking issue where I have to stand on the pedal to get the thing to stop and when I do stand on the pedal if I'm not careful the rear drums lock up. I was going to replace the booster but I'm not sure if maybe I just have the wrong proportioning valve or possibly both? I have front disc and rear drums but I'm starting to wonder if I have the wrong proportioning valve. I was thinking about installing a classic industries PB1108 black anodized adjustable valve, but I don't know if I should mess around with these adjustable ones. Just trying to save some money and not buy a whole new booster and valve assembly. Thoughts?

Truck is a 71 C10 with front discs and rear drums.