Hi Mike.
Be careful what you poke into the sensor hole, as any debris left behind even bits of rust can and will effect the magnetic ring, below are some photo's that I took when a member had drilled his sensor out, successfully and at first it cured the ABS light, but after a week the light came back on but this time he had the pulsing of the brake pedal.
We decided that a minute bit of the old sensor may have been left behind and damaged the magnetic ring.
I had a spare hub which we knew was a working one, therefore we made plans to change it over.
donor hub.
1
2
3
As you can see it is clean it was brushed out with a soft brush and also hovered so free of any rust particles.
The next day we remove troubling hub and magnetic ring, note how the ring as been worn by the debris.
4
5
6
You can clearly see the black protective membrane as been worn away so it is now showing the metal colour.
The donor one was fitted and is to this day still working fine, so yes bits can drop in and do damage best to be clean
Arctic