Soft pedal after hard turns

71 Duster. Did a SSBC 4 wheel disk brake conversion. Complete kit modeled on the Kelsey-Hayes calipers. 15" Rallye wheels. LBP.
In general, the car stops great & straight.
If I jerk the steering wheel, take an exit ramp aggressively, or hang a hard left or right, the first pedal push is soft and goes about 1/2 to the floor. After that, it is firm to the top and as long as I drive in a straight line, it remains hard.
So that means the rotor is flexing on the turns and pushing the pistons back in.
We have gone back through and checked for loose bolts, play ,wheel bearing tightness. The only thing we found was the wheel bearings were a little loose and one control arm was slightly bent. Fixed and was a smidge better but not much.
We've called SSBC and they are stumped.
The 30+ year mechanic I'm working with says this was a common issue on old Fords that used the same KH system.
I know something is flexing but am stumped as what to try next. Or is this just an inherit issue that is in these systems?
Any body else experienced this or have suggestions?
Thanks, John