Certainly with brake cylinders, I wouldn't change the seals if the old ones were already leaking, I would replace the cylinder.
I once and only once changed the seals on a mini, fitted new brake shoes and then two weeks later, had to replace the cylinders with new ones and get new brake shoes again.
However, I have never replaced seals on disc brakes, or even fitted new ones, so I can't say how effective it would be, but worth thinking about.
If the bores of the cylinder are scored or badly corroded, then new seals cannot be effective.
Don't try to do brakes on the cheap, more especially if used on public highways, unless you are certain that you know what you are doing.