When you rebuild your calipers it's best to grind a little bevel on the outer edge of the dust seal so they go in easily. Some of the calipers have a very tiny bevel machined into the start of the seal's bore but others don't and those that don't are real hard to get the seal started without bending the crap out of it.
I had forgotten about it when I rebuilt my front/left caliper several months ago and I ended up bending one of the dust seals so bad it took me over an hour to straighten it. But when I rebuilt my left/rear caliper yesterday I remembered to grind the bevels and the dust seals hammered in easily.
The dust seals often have a little "lip" on them which compounds the problem as they're almost impossible to install without bending them. Grind a little bevel on them first and that takes care of the problem.