Assuming you have no leaks, etc; it could be the caliper pistons are getting pushed back into the calipers while driving, then you have to move the pistons all the way back out when trying to stop and that will require 2 -3 pedal pumps. I look wheel bearing end play or excessive rotor runout that is knocking the pistons back into the caliper. notice if it does it after high speed cornering or left and right swerving. Good Luck