what do i need to buy if I want to upgrade the rear calipers to s4 calipers? can I upgrade just the brake booster or do i need to change the booster and the master? does the baster come with the booster or is it a seperate part? should I go with the newer a4 parts or go to s4 parts?

if someone has doen this upgrade please chime in.

Why to want to upgrade the rears to S4 calipers? Have you already upgraded the front, which always should be done first.

Why do you need to replace the booster and master cylinder? Do you have a copy the Bentley for the A4/S4? What are the pistons sizes in the A4 and S4 rear caliper? I can't believe that they're different enough to require a master cylinder change even if you do install an S4 rear caliper.

You are indeed correct. '99 and up A4s run the same master as the 2000-2001 S4.

I have Alcon B's up front and S4 rear calipers on my 98.5. The brakes worked with the original mc, but the pedal ended up waaaay too close to the floor for my comfort. If you go to the S4 master cylinder (4D0611021A) you'll need to replace the brake servo as well. It does bring the pedal back up where it belongs. S4 setup is a little "touchier" around town, but nice pedal feel for track use.

I like the setup. Be aware that the larger S4 master and servo took up some space that was needed for my strut tower cross brace :-(

I have Alcon type b up front and the car feels very unweighted upon hard braking on the track and I attribute it to the rear not being able to keep up with the fronts brake force. I've dealt with it for years but I want to do something about it. even with my current front brakes the pedal has a fair about of travel so with the larger s4 rears the pedal will be on the floor. I also have a friend with a new a4 and he claims the upgraded rears really helps keep the rear planted on the track and really improved the overall feel. I don't run real race pads on the rears because I think the small solid rear rotor wont cope with the heat and might do damage to the rear calipers.

The early a4s have a much smaller brake booster then the later a4s or the s4, and the early a4 doesnt have the displacement to operate larger front and rear calipers. If you have a idea on how to avoid upgrading the booster/mater I'm all ears, its going to be a very expensive upgrade for what I'm actually achieving.

I wonder if I can fit a larger booster on the stock a4 master? Would that improve the systems fluid moving ability or would I also need the master? I plan on using the ecs s4 rear upgrade with the 2 piece rotors

The A4's caliper piston sizes are in the Bentley, and I assume they're enough smaller to cause the pedal travel problem. I afraid I can't help with the master cylinder swap and defer to Spieks.

Upgrading the rear brakes, though, will do nothing to eliminate the feeling of an unweighted rear or help keep the rear planted on track. If you improve braking, you'll actually make the problem worse. Unweighting the rear is only a function g's of deceleration, the height of the car's center of gravity, and wheelbase.

Stiffer front springs can reduce the feeling of pitching forward, but about the only practical way to reduce weight transfer is to lower the car. Weight transfer is not affected when the braking is all handled by the front tires, rear tires, or a combination.

But I have a 97. Planning to change to the S4 caliper carriers and the S4 vented rotors. Keeping the same pads and calipers. Should be able to use the same brake master cylinder and booster. I know someone with this same setup and it works fine. He has StopTechs up front and I am running S4 rotors with Porsche calipers. (BIRA kit)
Just ordered the caliper carriers and the rotors today.