2002 brake pedal surge

I have a 2002 cougar with a 2.5 and automatic trans. The problem is I replaced all four rotors and pads and i still have a pedal surge only under light braking under hard braking theres is nothing no problem at all. I have a snap on scanner and there are no trouble codes in the abs system. what the heck is causing this? any help would be alot of help.