"A brake proportioning valve works by limiting the amount of of line pressure that passes through it. Typically, a proportioning valve is installed in the line which feeds the rear brakes. When the valve is fully open, there is no reduction in pressure flowing to the rear brakes. As the valve is adjusted, rear brake line pressure is reduced. This gives less braking ability to the rear brakes, and thus more braking force is available at the front brakes."
" A brake balance bar is a part which operates two master cylinders with one pedal. It mechanically regulates the amount of brake pedal stroke applied to each master cylinder. The pedal can apply force against an adjustable bar which inputs more force into one master cylinder than the other."
" Brake balance bars and brake proportioning valves both help to achieve proper front - to - rear brake balance for the car."
Quoted some quick passages from "Dwarf Car Technology" by Steve Smith. It's a good resource book, well worth the $24.95 price tag. There is a lot more info. I just posted the basic functions of each. Buy the book!
Hope that helps,
Steve