Testing brake bias on modern daily.

Hello! I was wondering if someone here could explain the signs of brake bias issues, aka failing caliper or hose for a modern. FWD ABS econobox. Google searches only seem to bring up racecars and bikes.

Did some high speed brake tests last night and found that my '13 optima feels pretty level and stable at light-moderate braking. During high speed heavy braking, she seems to nose-dive a bit. I understand this is normal due to weight transfer and I did not loose control of the car. When going backwards, that's when the rear end started to skid about. Maybe ~40 mph? No real world reason I ever would go that fast backwards.

I don't think there is any problem, just behaviors I noticed during maintenance/self inspection, and I was hoping to get a little insight just to make sure, and to knowledge myself for the future.

Hecc, since I'm here I'll also mention I have a '94 SHO with the rear proportioning valve completely bypassed and she is a bit tailspin happy during hard braking/cornering. I assume that means too much rear, right? (and rear left?)