my decals were hard to come off the backing, there is a technique to it, if you just press down and expect them to come off you'll find it difficult.
Its hard to explain, but the best way to do it is to carefully peel off one end a couple of mill, then stick that tiny bit down, and have the backing gently doubled over in your hand, allowing the backing to slip through your fingers in effect 'rolling on' the decal character by character, removing the backing as you go.
they dont need to stick very much to the caliper paint, just enough to stay in place so you can spray a few layers of laquer over them and jobs a good'un!
hope it helps!