[Mrmerlin]

from your info its time to make the brakes fresh,

replace all the flex lines with steel flex lines,
replace the MC and power booster and the valve that goes into the booster,

also replace the proportioning valves,
use ATE super GOLD fluid.

replace the calipers this should restore braking to full,

also consider swapping in larger rotors and calipers from an S4 on the front.

Dont cut any corners as it will mean you will be back at this issue within 1 year or less.