[dre99gsx]

That's correct - as long as I pump (enough times before a braking zone), braking is strong and catches up top of pedal travel.

It feels so much like pad knockback, except I've done everything to mitigate. 4lb springs in the calipers (these are the "top of the line" Stoptech Trophies btw), converted the Stoptech calipers to full floating with flat washers, new wheel bearings with proper nut torque, freshened up mostly all of the front suspension parts, etc.

The old calipers (335i stock units) were floating on brass pin upgrade, so because I didn't have this issue then, it further leads me to believe something is flexing in the front end.

For shits, I just tested my brake boost. Holds 30psi (check valve works), and holds 15in/hg vacuum for hour (no leaks in booster). I read stories of boosters acting funky.

If it was air trapped in the system, I don't see how I could build up pressure by pumping, it would always feel mushy no? The fact that I can usually get a solid pedal (sometimes I would forget to pump the brakes before a turn and surprisingly had a strong pedal) doesn't seem like air in the system.

Interesting thought about M3 knuckles... I may look into that further, worth a shot. Thanks for the photo references!

UPDATE: Ok - I ordered a used M3 front knuckle (cheap) with hub attached, will do some measurements and see what I can find. Wish someone designed a stronger version of the knuckle.