Hy
As I know,when B pressure low,a alternate source selector valve will open and system A will supply pressure to the brakes.Also a valve will isolate the brake pressure accumulator.
From brake schematic you'll see that fluid will go back to system A
It's normal,to avoid losing also fluid from A due to a leak in B sys.
But you are on to something.You can transfer fluid from system B to A
A (if i'm not wrong) by using brakes (on ground).Something like this -push brakes,than select hydraulics B off- then release brakes.The fluid will go to sys A. You can use this if you want to raise sys A quantity if a little below 88%.
You can transfer fluid from A to B by using the nr 1 reverser -normal open,then STBY retract.I'm not 100% sure,I have to find my notes,but i think I'm right
Alex