It's usually through the brake system, the 747 has the same procedure to pressurize the primary brake system first to prevent fluid transfer.
It's only a small amout but if done during every start will eventually require maintenance to rectify.
The reason is multiple sources are available to supply the brakes but ALL return fluid from the brakes goes to system 4 on the 747, I assume it is the RIGHT system in the T7.
So if we pressurize sys 1 on the 744 first a certain amount will go to the brakes and will end up via the return system in sys 4.