Ciper I am a perfect candidate to give actual experience testimony to everything you have posted. I just recently completed a drums to disc swap on my '01 Legacy. This was done first (using '00 Legacy GT wagon rear disc set-up). About a week later I installed Sti 4-pots on te front with WRX rotors. For the time being stock pads are being used and stock brakelines in the rear but stainless in front.
"First, drum brakes get an advantage over discs since the friction material is Wedged into the drum. This means for the same force you get additional braking power with drums (to a point)."
By feel I'd have to disagree. After putting in me rear discs my braking improved....although it was not a huge improvement it was better.
"Second is the amount of fluid sent to the drums. If you had a car setup with drums and an equal car setup with discs the drum brakes would get FAR less fluid for the same braking power."
I don't think so. You still have the same proprtioning valve so the same amount of fluid goes to the rear. When I installed the rear discs (still using stock single pot fronts) my brake bias transferred towards the rear. Nose dive lessened amazingly.
"Third is a valve that is present on disc/drum combo cars. At the moment I forgot the name(Im not referring adjustment of bias). In drum brake vehicles the pads are drawn away from the drum by a spring. Discs are basically in constant contact. Becuase of this a valve is installed that will send brake fluid to the rear FIRST, activating the rear brakes a short amount of time before the fronts. Since the rears need to take up the slack this pause usually means both brakes are activated at the same time."
The rear brakes are not activated before the fronts. With the disc install I still have consistant brake application. There was no sense of the rears kicking in ahed of the fronts. Even under very hard braking where the abs kicked in there was no notice of that.
"Brake power to the rear is GREATLY reduce for two reasons, the large increase in piston size compared to the drums and the reduced force needed to normally activate drums. "
Absolutely not. As I said my braking power increased as did rear bias both of which discount that.
"It seems to me that you would need to change the entire "chunk" of components that include the bias/proportion/timing valve in order for it to function correctly."
Nope. All I changed was the e-brake cable, backing plate, rotor and caliper. I ended up needing to change the hub as well since my drum hubs were stuck on due to rust. There was no change in master cylinder, or proportioning valve.
"I would think that some of you have been driving around with basically no rear brakes."
Not at all.
"Lets just say for a moment that is not as large an issue as I think. You still have the problem of the rear brakes activating way too early."
Nope, I'm curious what gave you these ideas. Although you did give your reasons there doesn't seem to be any actual experience with this. (Which I suppose is why you are asking).
I now have the Sti's on the front and there is still heaps of rear brake. Except under very hard braking there is little nose dive (and I am on stock suspension at the moment). I had my car on the auto-x course this past weekend and the breaks are great. The car stays far more settled under braking and braking power has improved tremendously.
I can't wait to get some stainless lines and better pads and a master-cylinder brake from MRT.
Hope this helps with the quest for info. It's just one persons opinion but if you ever find yourself in Vancouver you are welcome to test the car out.