1990 Dodge Dakota 2wd ext. cab v6

My brakes are really spongy. I just replaced the front rotors, and pads, the reservior is at a good level, but while at stop lights, my pedal slowly depresses to the floor. While driving, they are tight, but when stopped, and especially when stopped on a downgrade, the brake will be fine at first, but then, you can feel your foot slowly creeping toward the floor. I haven't found any leaks, and like I said the reservior looks good. Maybe air?