Classic slide pin or one of the pistons isn't working in the front calipers. However if its both sides the likelihood of both calipers being bad is low. Slide pins routinely get gummed up tho if they aren't greased every time the brakes are done. I always do. Do the brakes, use good rotors like Raybestos, make sure you compress both pistons properly, remove and grease the slide pins, new rubber boots if you need them on the pins and then bleed the brake system.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.