locking brakes

i have a 02 reg octavia sdi and the front brakes are locking on after around 10 mins of driving and the brakes warm and the front wheels lock up, i have checked the calipers and greased the slides,and today replaced the master cylinder and they are still the same but when i opened one of the bleed nipples on the master cylinder they released themselves has anyone got any ideas what it could be
cheers in advance guy

ive had the flexi brake pipes break up inside try loosening the pipe s at the top joint to confirm if this frees brakes then change flexis

thanks will do