I have some Hi-Spec Monster 4 calipers with 300mm discs and have fitted a 75 Twin Spark master cylinder. As mentioned above you do need to modify one of the brake pipes a little to get it to fit but other than that its a straight swap. My brakes probably need bleeding again as the pedal travel is quite long before the brakes begin to bite, a little disappointing at the moment because I was under the impression that the upgraded master cylinder would avoid this. Hopefully another bleed will improve things.
Hi, if you have this on your car:
Monster 4 Radial to Suit 300mm Rotor
the area of pistons on front calipers is now 1,4 x original area
(at the front, I dont know what mod have you done at rear),
so it is possible the 75 master brake pump is a good mod
(it move 1,16 x more liquid than original brake pump from 33)
but it is still not enough to fully compensate..
I hope with another bleed of the system you can gain some
shorter travel..If you have drums at rear another possibility
is that rear automatic play regulators inside drums are
not working (very common) and you have to regulate it by hand,
have it regulated usually short the pedal travel and improve feeling..
Thinking about..maybe when installing big brakes
(rotors and calipers) if possible one should prefer calipers
with pistons not too much big, this to reduce the other mods
required to the car and to keep a more equilibrium
with rear brakes?
If you doesn't care about modify original pipes,
one another possibility is use brake booster and brake pump
from other cars, I'm trying this, but it is quite complicate/time
consuming, for the mods to the pipes, probably an overkilled solution..
and not legal in italy, so it can't be used for long time on a street car.