Well, that leaves the master cylinder to give a squishy or hard pedal.I'll check that now, but my reservoir hasn't shown any signs of missing brake fluid. Also, all 4 corners have brand new OEM brake calipers. The LSPV was also replaced with a brand new OEM one.
I'm still leaning towards the booster, though.
It took mine a week of DD for it to go rock hard 100% of the time.