Testing a Proportioning Valve?

Antilock brake system is the only true way to prevent rear wheel locking under hard/emergency/panic braking. The fronts are supposed to roll / be steerable after you're dragging the rears. Lock all 4 at once and you're sliding with no control. If rears lock under normal/light braking, arc or radius match of shoe to drum is possible cause. Shoes installed wrong is another cause. The shorter brake lining goes to the front.