Old drain patch fail

We do a lot of hidden or concealed gutter work. Especially at the beginning of a roofing season as weather and temperatures limit our work “windows” on these jobs until mid-May. What we always find is that the rubber gutter liner is never done up to today’s accepted standards. They typically fail and leak at corner seams and drain protrusions. We were in Burnaby today doing a complete upgrade on an EPDM rubber gutter system. It was not any different then most other membrane gutter work we do. The attached photo shows the ease of which I am able to remove old patches from the copper drain hardware. I don’t know what the original installer did or didn’t do for it to fail like this but I see it in most homes I do this work on. Either he used the wrong glue, maybe didn’t glue both surfaces, didn’t clean them properly or with the correct cleaner. Whatever the case may be, we know it’s been leaking for a while as there is always rust and debris found below the failing patch. We need to remove the old and install a new. Come to think of it that’s something I’d like to do with my hair.