Ken,

I'm sure you agree that locked brakes don't slow the car as quickly as ones that permit the tires to turn just a bit underspeed. If the fronts lock prematurely relative to the rears, you're not getting full braking from the rears -- and that's significant, even though most of the braking is being accomplished by the fronts. There's not just there for ballast; the car would be fully stable in a straight line with no rear brakes at all.

When mods increase the effectiveness of the front brakes, it's therefore also necessary to provide a matching increase in the rear. The simplest way to do that is to divert hydraulic pressure away from the fronts and send it to the rear brakes, and an adjustable proportioning valve (or possibly alternate limiting valves from a different model, as Jack suggests) is the way to do that.

There's a lot more to the subject than this thread will bear -- the optimum biasing varies with speed, for instance -- but the short answer is, Yes, I am sure.

There's an excellent article on brake biasing in the current issue of Grassroots Motorsports, BTW. Check it out if you can get your hands on a copy.

--Phil