Front brakes always do the majority of the work and with a front rake, they do even more, like 60% -75% of the stopping. That is why you can usually do one rear wheel brake job for every two front jobs.
Your goal is to have both front and rear lock up on a panic stop at near the same time with the fronts locking up first. This will give you the maximum available stopping power with a given system. The only way to determine if you need a proportioning valve is to go to an isolated piece of pavement and lock up the brakes from a 20-30mph speed (slow as possible to be safe) while someone watches from the outside. If they lock up as described above, things are great! If the rears lock up before the fronts, then you will need a proportioning valve in the rear brake line so you can reduce the pressure to allow the fronts to do their job. NEVER put the valve in the front line - this reduces the overall system efficiency and is dangerous. If your rear brakes are WAY too big, they will lock up way too soon and your car will instantly try to swap ends so be careful! My Willys was set up like that when I was young and dumb and with my slick rear tires, it was an E-ticket ride at at stop signs on a rainy day! If your fronts lock up and the rears never do, you will need to install the next larger diameter rear wheel cylinder to give them more stopping force. If that then makes the rear brakes too strong, again a proportioning valve will fix that.
All sounds complicated but it isn't. Just takes some busy work to tune the system and it is done for life.
[ March 25, 2003: Message edited by: firstname.lastname@example.org ]</p>