Considering you're building a Kart on steroids (no offense as I love 914's) I prefer the raw braking modulation a manual master cylinder delivers. Coming from a motorcycle background non-assisted brakes just feel proper to me and add another element of driving challenge.
I won't argue the benefits of ABS and on the track a good system will beat a human but where's the fun in that!
From what I've seen the systems that really work on the track without faults are not inexpensive.
