What is the 'fuel cost' of a pneumatic braking system vs. a hydraulic one, in terms of weight? A pneumatic braking system has indisputable advantages over a hydraulic braking system for large and heavy aircraft in terms of braking force and secondary perhaps in terms of weight (pipes and hydraulic fluid). However it requires engine input to compress the air and generate the pressure and force required for braking, all of which use fuel. 

What is the cost of a pneumatic braking system from a fuel point of view as opposed to hydraulic in relation to weight for a large Jumbo like the Airbus A380 or Boeing 747 if one had to consider using hydraulic brakes for any of these aircraft?
There is an existing triple redundant hydraulic system in those planes required used for the control surfaces. Those are not something you want controlled by pneumatics.

This means the weight of the compressor, buffer tank etc. is already there and using it for brakes means you only need to pull a line from a supply/return line (which would also be near the gear to actuate it anyway) and adding a servo valve.