ziggy:
the system was designed to work automatically in the most critical situation possible: engine failure after V1, or during go-around.
In such a case it's criticall to raise the gear asap,so if LGTU senses low eng1 RPM ,thus low output from eng1 hyd pump (even if sysA elec pump it's still working ---it's not a loss of hyd sys A ) then sysB supplies the hyd volume necessary to raise the gear at a normal rate.
Now,you may say what if you have a loss of sysA during take-off.You'll still have 2 engines running,a less critical situation during t-off,giving you plenty of time to asses your situation after climbing to a safe altitude.
You can contact maintainance,find out what CB to pull,or decide to shutdwn eng1,whatever.It's almost no risk in shutting down and restarting an engine,if it's done properly.
Brgds Alex