Here's the URL to Automotion's brake performance page. On the very bottom of
the page, you will find something called a "914 racing brake balancer". This
is a proportioning valve in the true sense of the word. I hope you are not
referring to the brake pressure regulator. It's not cheap but it does work
very well.

http://www.automotion.com/am/page_listings9.asp?c=20700&p=30335

There's also an outfit called TSM who makes a less expensive version. I will
try to find a website for them and post it here.

BenT
San Francisco