Right - got the SLS back on and bled last night no worries.
So, me and my pal decided to do the rear brakes as well - the system at the back was empty, with fluid up as far as where the front-rear pipe turns up into the axle tunnel.

So....

We spent approx 40mins bleeding the system - plenty of air coming through - but no fluid?!?!

And it wasn't drawing any fluid off from the reservoir!

And it wasn't leaking anywhere

Totally stumped me and my mate (both experienced driveway mechanics)

Any ideas anyone?

The only thing I could think of was the reservoir/master cylinder seal gone and its drawing air in?