It is NOT the extra weight of a cast iron m/cl that the brace is reqd. The difference in weight between a CI & an alum m/c would be lucky to be 4 lbs......
That is nothing compared to the force of you number 12s on the brake pedal.....
That is what the brace is for.....
if that were the case (force from braking) then why wouldn't a manual master cylinder have bracing? ostensibly, that would be transferring even more pedal force than the power booster directly to the firewall.