The extra braking energy is stored in the silver or gold colored cylinder that is part of the hydroboost unit itself.
There is no "nitrogen ball", at least not on the systems from Hydratech.
I'm sure a nitrogen-charged accumulator could be added if one really wanted it!

I think that getting an all-new system for a little more than $500 is pretty good. Or you can cobble one together for less from used parts.

Compared to the grabby characteristics of '70s Chrysler power brakes that I remember from back in the day, the Hydraboost is a dream.

R.