I tested two 2003 A8 - the braking puzzles me => on a dry asphalt the cars are leaving rubber marks on the road.

The ABS sounds, the light comes on, no DTC.
My believ is that ABS function is to prevent locking.

Is there a way to do furter test, or is not worthy to pay attention as the ABS are toasted?

 

The ABS locks the wheel and releases it many times per second, it will still seem like there is a solid skid mark by looking at the asphalt. ABS is worthless in my opinion because on slippery roads it actually increases stopping distance by causing the car to slide several times over, gravel especially.
How do you know your abs is toasted? The light is supposed to turn on when the ABS is activated, thats a normal condition. No DTC's will be stored when the ABS activates.

 

Then the condition is a normal one.
On my experience, not all abs leave skid marks on the asphalt.
The cars that were leaving skid marks were dying on steep slippery surface, spinning all four wheels.
By design, I guess.

Thx,