I have rebuilt many master cylinders. The seals are a pain to do, however there is a method and once you know how to do it it’s not so bad. You need to soak the seals in hot water until they become soft enough to fit over the piston without tearing. This could be 5 mins, sometimes 10 mins. The grease in the kit also helps with this. I find it helps to clamp the piston in a vice so you can use both thumbs to push the seal on. Using tools will tear it.
Chris