If you take the pad's out and the pistons really need to be forced back in like real pressure to force the pistons back you will know that's the problem. You can work them free but from experience is will come back. Check for any split rubber around the pistons too, will almost course issue at some point. It's the easy way to work it out if that's the problem or might be early signs but working them might give you a few months if nothing else all worth a try if it's free