When does a leak become the responsibility of the association

If there is gross negligence involved the party being negligent should be held responsible. You would have to prove gross negligence and it is probably easier to just let your insurance company deal with it but that costs you the deductible and maybe your rates might go up.

Check your documents to see exactly who's responsible for what - depending on what it is, the owner may, in fact, be responsible. You may also need to check with the association insurance company as well.

As for the management company, they can be replaced by the board, who oversees the overall direction, while the management company handles daily operations. It's not about micro-managing but making sure things are being checked regularly. Since the board oversees the property manager, it's up to the people who oversee them (you and your neighbors) to hold them accountable - if they aren't doing their job, check the documents again to see what it says about recalls or calling a special homeowners' meeting to do it. You will, of course, need other folks ready and willing to step up and take over - and one of those may need to be YOU.

You also mention negligence - if there have been ongoing problems with the boiler, plumbing, etc., has anyone inspected it 9them?) to see what needs to be done to fix this once and for all? There could be a money issue that needs to be addressed and hasn't been (because repairs and replacement ain't free). Usually, the association has a reserve fund to address replacement of major components and if you don't know the current state of yours, it's time you asked.

I don't see a case for "gross negligence". Response time varies and not guaranteed. Doesn't make it "gross negligence". Especially if maintenance is handled by off-site. A few hours could be expected for response time.