You need to be aware of where the fluid went and why the system failed too, but there is no
need to 'return' to departure. While it
may be the most sensible in terms of weather, fuel, maintenance etc etc, flight could perhaps continue either to destination or a more suitable diversion airfield if conditions are suitable. Obviously your max altitude is limited and drag is higher with the LEDs out so care is needed, and you are, unfortunately in 'uncharted territory' as far as the FMC fuel predictions are concerned, so it is back to a 'how-goze-it' chart on the back of that well-know fag packet. Likewise with sys A - there may be good reasons to 'return' - and not.
Especially in the sim (Sys B), if everything else looks ok, I would tend to continue en route at a lower level while you carefully and thoroughly exercise any one (or more) of the 20 or 30 different favourite 'problem analysis' catchphrases.
. That will also give you a bit of space to rebrief a return - if that is your ultimate decision - without being 'rushed', plus a nice measured 'NITS' brief and perfect pax PA
. Don't forget to try your 'synthetic' calls to company for 'ops/engineering' advice as well for max brownie points but beware the 'gotcha' reply you may be thrown - it must be your/Captain's decision in the end.