I started to notice that brakes were spongy. After 700 miles with ebc hh pads. So I bleed the brakes with brand new dot 4 fluids . Brake lever felt nice and strong now. After 10 mins of riding it went back to spongy again. What could it be? I have the ztl2 8 piston brakes.

Could it be that I need to replace the steel brake lines? I did see a small abrasion near the front fender tire rubbing. It rubbed the outter pvc shield of the cable but the steel braids is fine. Do you think this could be the problem?