Yes you need a bias valve if its the same brake setup as Tony........

Picture the sceneIm driving Tony's car M11... 120 - 130mph, right behind a Honda civic / accord IIRC..... for no reason whatsoever, he decides to brake???!!!! My reaction was to do the same obviously, at which point the N/S rear wheel decided it actually wanted to be an O/S front wheel!!!! The back end Properly tried coming round, which wasnt a funny experience at the time in an unfamiliar car!!!!!!! PROPER brown trouser moment !!