Rhys,
Air in the system can in fact cause block peddle mate,
I understanded where you are coming from,
But to discount air from the system completely at this stage, may in fact lead you down the incorrect path.
There is a sequence that you have to bleed the brakes with ABS in a skyline if it is to be done properly.
There is write up for this in the GTR Manual that can be found online.
Once you have cleared the system completely, then can you move onto the next fault finding excercise.
Nav had a very similiar issue with his car.
Cheers
Mike