QSC (Stourbridge) originally said not to fit the G-valves, but to change the rear cylinders to a different (smaller) size. These were supplied in the kit and varied with front brake and A or B series brakes.
Later they sent out an amendment (August 1993) which reiterated the correct size of rear cylinders to use for different versions of the car and that from August 1993 all cars would be built with the G-valves in place. They also explain how to fix the G-valve if you wish to, but it is only a recommendation.
I spoke to the factory about this and it seems that some people had experienced rear wheel lock up and the cars had spun and the main problems was replacement (service item) cylinders were fitted or the wrong size. Quantum fitted smaller rear cylinders to get the balance correct, but later on bigger cylinders were fitted by garages.