i have noticed that there is a not a lot of collected information about the stock FC brake system, and maybe it seems complicated, as there are a lot of different models that do different things in different years. i actually found the braking system to be pretty simple, thus;

Front knuckles: there are two kinds. ones with the holes for the ABS wheel speed sensor, and everything else. only difference is the spot for the sensor.

Front wheel hubs: there are two. 4 lug and 5 lug. the 4 lug only occurs in the US on the base models

Front brake rotors: there are two, 4 lug and 5 lug.

Front calipers: the 5 lug S4 and upper model S5, and every JDM car gets the 4 piston calipers. the 4 lug S4 cars get a single piston, and the base S5 cars get a single piston.

Rear caliper: this is the easy one, there are two. one for the 4 piston cars (all JDM, every 5 lug S4, and the upper model S5's), and then one for the non vented base model brakes.

rear rotor: there are three. 4 lug. then the S5 base models use the same 4 lug rotor with 5 lugs, and then the rotor used with the 4 piston calipers. this one is vented, the base brakes are solid.

proportioning valve: every car gets the same one.

Brake booster: there are two of these. there is one that does ALL of the S4's and all of the S5's except the turbo. the second one is the S5 turbo car, and all the JDM cars

Master cylinder: this is the only real tricky part, there are 6 different ones.

S5 turbo WITH ABS.
S5 turbo withOUT ABS, which is not US spec
S4 GXL, this has a brake pressure sensor for the AAS system.
S4 GXL with ABS, this has 2 lines for the ABS and the hole for the AAS sensor
S4 turbo with ABS, this has 2 lines but no AAS sensor
and then there is one that does everything else.

all of these masters, except the S5 turbo are the same size. the S5 turbo also uses a different mounting, which was subsequently made standard, so its shared with the FD/Rx8/929/and most other cars.

 

Good info. I had to replace my booster and MC on my S4 GXL. Subsequently I did not retain the AAS system at all since everything suspension related had been upgraded anyhow. Have not noticed anything "off"

 

Great info! I'm guessing that the sport model is considered a base model? As I have an '87 sport with four lugs and single piston brakes