I would say from what you are saying a damper might of given up meaning one of the wheels is pattering (bouncing up and down) meaning the ABS kicks in early. The senors themselves are either a sensor counting the ends of the wheel bolts going past or more commoningly a toothed wheel on each hub with an inductive sensor next to it.

Clean up the toothed wheels but try not to move the inductive sensor as they are distance critical.

As to tweaking the senstivity of the system that is as far as i know software controled But other things that might cause early lock up are, bad alignment, too high tyre pressure, crap tyres (nasty high milage ones).

To check the dampers it is more experience etc but bouce the car at each corner and see what happens if the car wobbles alot it might be a screwed damper. also the handling might be a bit on the [censored] side causing it to bottom out early and other nastys.

If you feel brave you could on a quite road pull the ABS fuses and try the brakes and see what happens if it locks early or pulls or some other nasty just do it at low speed and be ready for something nasty to happen. I would try that but i am used to driving vechiles that do strange things after owning some real garbage.