B777 Hydraulic Fluid transfer
 

Hello ello,
This may be a completely stupid question, however, i cant seem to find the answer. In the CBT and tech manual it says to pressurise the RHS hydraulic system first, and depressurise it last to prevent unwanted fluid transfer. I've been looking at the system diagrams, in both cbt and tech manual, and can not see any physical connection between the systems. How can fluid transfer?, if fluid can transfer what is the purpose of designing it that way?
Thanks a lot.
Seabiscit

Probably something to do with the Primary and secondary brakes, I assume the primary brakes are powered by the Right system.

SMOC

The normal brakes come from the right hand hydraulic system and fluid can transfer across the brake shuttle valves. Pressurising the r/h side first and depressurising last stops the brake shuttle valves from moving across and allowing any (albeit small) fluid transfer.
If you set the parking brake in one of the conditions below, and then release the brake with only the center system pressurized, the fluid can move to the centre system:

(1) The right and centre systems pressurized

(2) Only the right system pressurized

(3) No hydraulic system pressurized.

If you set the parking brake with only the centre system pressurized, and then release the brake in one of the conditions below, the fluid can move to the right system:

(1) The right and center systems pressurized

(2) Only the right system pressurized

(3) No hydraulic system pressurized.

To keep the movement of the fluid between the right and centre hydraulic systems to a minimum, the conditions which follow are important:

(1) Pressurize the right hydraulic system first.

(2) Remove the pressure from the right hydraulic system last.

Seabiscuit,

I don't wish to hijack your thread or anything but I thought I would ask this question here as the thread would attract B777 afficianados.

Does anyone know of a good 'tech' website for the 777, along the lines of the very good sites such as:

www.airbusdriver.net
www.b737.org.uk

These are run and contributed to by some very dedicated folk and I wonder if a dedicated 777 team has done anything similar.