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UPDATE:
I ended up finding a leaking brake fitting right on top of the ABS module - it would weep as I pushed down on the pedal with the vacuum pump on.
I fixed it, re-bled the system, and the issue still persists. I have a new theory.
On the track, I'm never more than 13in/hg vacuum on decel. With the vacuum pump, I'm at 25in/hg, which is probably providing more brake assist, thus probably why I'm able to have the brake pedal drop this much.
I dialed the vacuum pump back down to 13in/hg and it took more force and longer time for brake pedal to drop.
I checked every fitting, no additional leaks or weeping found. I did yet another INPA abs prime and powerbleed fluid, no change. I read that the N52 vacuum pump provides 25~30hg/mm vacuum at all times to the booster? I'm directly connected to the intake manifold thus it only pulls vacuum at idle/braking. Engine is cammed and has low vacuum at idle by nature (around 8hg/mm). At boost, the check valve on the booster blocks positive pressure (tested that as well).
I am now suspecting this may be part or most of my issue. I'm on the brakes hard for 3seconds? or so per corner, which then my pedal sinks a bit from just standing on it, and my pedal now is lower than before for engagement on the next corner REGARDLESS if I was turning or not. I can test this on the highway to confirm, but it makes sense.
If you guys know my pedal should not sink with engine on/vacuum applied, then the only thing I can thing of is the ABS module is leaking fluid internally, and needs to be replaced (I'm on my second one already from the trifecta-of-lights issue). I'm going to buy plugs for the master cylinder and isolate the system part by part, see if I can find something...
Master cylinders are now $5xx (was $3xx), not looking to replace a new one I just installed.
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