Mainly when it rains (it has done it when not raining) when using the brakes the ABS is engaged. Most of the time the light comes on also. What little I know/heard is it could be a cracked ring. What can I do to troubleshoot this? and are there any pictures of this "ring" installed? It's my sisters car and she lives 3 hours away.
Thanks loads

What year?

Find out what the codes are and focus your attention in that area. If it acts up more when it is wet I would be more inclined to think there is an electrical issue rather than a mechanical issue. Wheel speed sensor, corroded connectors, corroded wires, etc.

A broken exciter ring, usually located on the drive axle, typically causes an overactive ABS when stopping at slower speeds. A quick inspection, if its located externally on the axle shaft, will easily spot a cracked one as it will literally be cracked, this causes one spot in the exciter ring to have more space between the teeth. More space = longer time between signals = fooling the ABS module into thinking the wheel is turning much slower than it really is when it gets to that spot.