In an all drum set up there is no proportioning valve. There is a safety valve, which is available from Doctor Diff.

http://www.doctordiff.com/brake-valve.html

If it's manual brakes you need to swap the rod over. You will need a new retainer.

Might ask Doc if he has any retainers separate, I know you can get them with his adjustable pushrod kit.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.