Binding front brakes

[Arbie]

Hi there. The front brakes on my foxy have just started binding slightly....not badly mind but the disc gets slightly warm to the touch even when ive tested it by riding for a few miles only using the rear. I've done the obvious of thoroughly cleaning the pistons and they all move freely using finger pressure which is why this is a headscratcher. I wouldn't have thought the seals would need replacing as the bike has only covered 20k. Any light shone in my general direction would be much appreciated. Thanks