Originally Posted by MUDCOWS

Resetting your pistons may help to resolve this issue or you may have a sticky piston on one side that is causing this issue. Remove pads and use a piston press or a plastic coated cone spanner to push your pistons all of the way into the calliper, pull brake lever very lightly to see if both pistons are moving freely and equally (be very careful not to push pistons all of the way out). If not clean pistons with isopropyl using cotton buds, then with a cotton bud dipped in brake fluid smear around sides of piston. Hope this helps.