When I originally did the conversion, I did not change the booster/mc/proportioning valve. My car did not stop all that well and it definitely wasn't as good as the stock single setup. I did bleed the brakes several times but no change. It was only when I swapped the rest of it that the car finally stopped properly. Not everyone has had that experience but I did so I'm not too sure what to tell you.

 

I thought my S4 SE stoped bettor then my S4 GTU