2002 Chevy Astro ABS light wont go off

I recently replaced front rotors and pads and 1 year later the abs light was coming on for days at a time and every now and than it would go off. No it stays on steady and so I put new drums and shoes and hardware on the rear and it had no effect.I checked the fuse and it is good.I have noticed the master cylinder gets low more sonner than it used to. But I dont see any leaking fluid on the driveway from it and there was no wetness in any of the brake shoes or pads. Can you help. Thankyou ron