[harnishclan]

So where in the rule book does it say that using a GT3 master cylinder is a legal SPB modification?

5. Brake System
A. Brake pads are free.
B. Steel braided brake lines are allowed.
C. Emergency brake, lever, cables and associated parts may be removed.
D. Brake cooling systems are allowed if they use only air. Air may be vented through the front air dam. Dust shield may be removed.
E. One piece stock size steel rotors are required. Rotors may be cross‐drilled or slotted.

SPB Class Rules and SPB Eligible Models
All parts must be factory stock from one of the eligible years, except where modifications are specifically allowed below. Modifications not specifically listed are prohibited.