Brake pedal soft

What's going on idk if anyone uses this forum but I got a small problem I noticed my brake pedal seems to go way far down then actually starts putting pressure on my brakes the driver passenger side rotor has rust on it were the pad seems to go over. Like it's not grabbing that one part of the rotor the drivers side is is perfect no rust at all idk if it's the caliper or not the car doesn't pull to one side while breaking. Brake booster holds hard and builds pressure when car is off and stays hard while holding down the pedal. I've bled my brakes still does it was thinking bout just changing my rotors and caliper to just eliminate some possible causes. Thank you