Very early cars had 0.75" brake master cylinder and 0.75" rear wheel cylinders, but this was changed to 0.7" brake master cylinder and 0.7" rear wheel cylinders. I believe this was due to rear brake locking problems, but even with the correct 0.7" master and wheel cylinders fitted, my car had too much rear brakes and was prone to end up sideways in heavy braking situations.
Tommy