In-Car at Willow Springs + Off-track dirt action...Thanks Ice Mode...

Dwnshift

ABS block/pump
Vacuum assist brakes need vacuum operate the ABS... If there is not an ample amount of vacuum to supply the system to operate... You will not be able to compress the pistons of the brake calipers.. Giving the driver a super hard pedal.
To me... That's why I don't recommend super double throw down brake pads for tracking. Hugher clamping force will trigger ABS to cycle /use more vacuum...big slow hard hits in the pedal. Then there isn't enough time to build up the supply for the next corner... and it starts all over again.
The OEM ABS computer is not programmed for high sampling rates and cannot keep up with the short supply. A Motorsport ABS pump and computer has much faster sampling and cyles...and the pules felt in the pedal are much softer shorter and faster.... More efficient use of the vacuum supply.
The use of a vacuum accumulator or reservoir can help because it allows vacuum to build up giving the ABS a larger supply to feed from.
That's my version.