Rear discs willpay off. Since discs are much easier to modulate, the rears will not lock up like drums in a panic stop. Just keeping the rears unlocked, will shorten stopping times dramatically. As far as assit goes, I have never seen a vacuum can do anything. If it worked, the factory would use them. Right now it seems the only 2 good alternatives are an electric pump (I use an old GM style, very fast and quiet) or a hydroboost setup. Both of these mimic factory configurations.