my abs keeps activating?

helppppppp i have a peugeot 307 1.4hdi and all of a sudden it seems that when i brake the abs is kicking in and the peddle pumps up and down.i have replaced the front discs and pads but it still does it.the abs light does not come on to say there is a fault.i have read on the net it can be as simple as corrosion around the sensors.so should i test them with a multimetre or just take the sensors out and clean them..please explain as best as possible and i am very gratefull of any info