Had same thing on a 88 f150. Just had brake work done and pedal went to the floor. Took it back and they bled the lines again. Still did it. Then they said it had to be the rubber lines in the front. Seems they break down inside and get little flaps that will make the bleeding process not as effective cause they will catch junk in the lines. Replaced lines, bled again, never had problem again. If I recall it was under $15.00 per side for the lines.