Weak Brakes - Master Cylinder?

[aerosam]

Hi guys - had my car out on track yesterday, and the brakes were pretty lacklustre. It's all working correctly, stops in a straight line,they're just a bit weak, I really have to stand hard on the pedal.

They are sierra fronts and BMW rears (discs). The rears are huge so power is re-directed forward by a bias control valve. The master cylinder is out of my donor BMW E34 5 series, and it had a hydraulic powered servo, which I didn't use as it was very long and the hydraulics were complex.

So, I'm guessing the unassisted master cylinder simply can't provide enough stopping force. Isn't the roadster normally fitted with a sierra master cylinder without the servo? Is there room for the servo, if required?

I'm thinking of replacing my donor cylinder with a sierra one, with servo, in order to seriously upgrade my braking power. Unless you guys can think of anything else that would be hold the brakes back?