Update:
All fixed. I ended up replacing the master cylinder, and after a lot of "soft pumping", the rear caliper ended up acting completely normal.
I had checked the other calipers and they all bled fine, it was just the rear left that would slowly trickle, and I think in my frustration, I damaged the master cylinder. A $60 MC later and a few litres of brake fluid, and it's back to normal. ?
On a side note, looking from the top of the engine bay, this wire is sitting on top of the steering rack. It wasn't there last time I looked. Anyone got a rough idea what it's for?