1. Sys B has a standpipe which holds approx 24% of the system fluid for PTU operation of LE slats, flaps and autoslat. If the leak is in Sys A you will lose all the fluid as there is no standpipe there.
2.The Mach trim system defelects the elevator; this creates a force on the elevator. That force is in turn creates a torque which is sensed, I beleive,by the feel computer which generates a sugnal to the stab trim motor to move the stab to zero the torque and hence the force. Thus the stab "follows up" the elevator.
3. The bugged speed is the Vman for the flap setting i.e at 210 kts Flap 1 is selected, fly the 1 bug. When Flpa 5 is selected, fly the 5 bug etc.