SSBC disc brakes won't stop

Pulling my hair out on this brake system. SSBC disc brakes front and back. 4 piston kelsey hays type ssbc on front and single piston calipers on the rears. Replaced the 1" Master with a 7/8" wilwood master to increase pressure to the calipers, but didn't seem to change a thing. Brake light coming on now when I jam on the brakes. Why would this happen?