Union would not pay for my 8 weeks sick leave - what to do?

It doesn't matter whether YOU say it's work related; it's what the union determines it to be. I'm guessing that you're an ignorant dupe who believes that your union exists for your good. Anyway, once they determine the injury to be job related (which, your being a janitor, it probably was) it's a workers' comp issue, not a sick pay issue.