Q: ABS noise, brake pedal sinking to floor on 2001 Chevrolet Prizm

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ABS activates on dry roads during gentle braking - pulsing brake pedal.

Brake pedal goes all the way to the floor, but can be pumped up - no loss of brake fluid.

ABS light comes on - very occasionally - maybe once a week. Resets to correct operation (Shows at engine start, than goes out) at next start up.

Car has been to two shops, including official Chevy Dealer, without a solution.
(1) Answer
For pedal to drop would need hydraulic failure and if not losing fluid would be internal bleed by at the brake master cylinder likely. Would feel more comfortable about answer if I knew this system better. Used to be that some abs systems had a very confusing/expensive combination abs unit and master cylinder set up but not sure if yours does. Hopefully someone will answer that knows for sure.