One or both of your rotors can be wrapped enough that while driving, the rotors will push on the brake pads causing the caliper pistons to push back. This causes a larger gap between the pads & rotors, which causes you to pump the brake. I had this problem on a 77 Dodge 4x4. I bought the truck that way & it was scary to drive but it didn't take long to figure out. Sometimes you can feel the pulsation in the brake pedal while applying the brakes. Take off the front wheels & step on the brake. Then try to rotate the hubs by hand & see what happens. If your rotors have deep grooves already, buy a new set & have them turned so they're true. Alot of people think that a new rotor is true but some still have what is called runout. The runout is what you want to get rid of.
