disco intermitant sticking brakes

help please I have a problem with the brakes on a disco td5 2002 brakes have taken to sticking not same wheels all times can be just rears or fronts or any three or all, I get no amigo warning lights (other than switch on when they light and go out as is normal) computer plug in indicated a shuttle valve switch fault but everywhere I look this fault should have the amigo warnings as well which I dont get, pads renewed no problems with the calipers pistons found.any ideas would be welcome

Try the flexible brake pipes they perish from the inside n make the pipe smaller n make your brakes feel like they are sticking. They stick for a few seconds then release this is due to the inside of the flexible pipes not being able to release the fluid as quick
Neil