Unless the contents had gone completely (unlikely)
Why is it 'unlikely'? I had a split hyd line in a 767 and it emptied the entire contents of the hyd system over the ramp in no time at all. By the time the warning came up, we'd had a look at the contents reducing on the Status Page and then turned the pumps off, all the fluid was gone. It doesn't take long at 3000 psi.
why they couldn't switch the pumps back on and accept a further contents drop till the wheels were locked?
(a)See my suggestion above (b)if you're going to rely on 'stand pipe' hyd fluid then you're out of luck, since the 767 C system reserve fluid only supplies brakes and steering and relies on the Altn system for gear lowering.