here is my setup. i believe i have front pads from porterfield and stock rotors that have been turned once. the rears are all stock. at 60k ive never changed the brake fluids.

problem: at certain times the brake pedal is VERY mushy. its not smooth at all. everytime the pedal feels mushy, i hear a squealing or grind noise in the front.

anyone have some insight on this? i know it can be caused my warped rotors, air in the brake lines, or maybe even the harsh brake pads.

questions:
-how can i tell if my rotors are warped?
-how can i tell if i have air in the brake lines?

thanks.