Plumbing problem in co-op apartment - what legal recourse do I have?

I have a plumbing problem in my co-op apartment. I sometimes have to let the water run for 5 - 10 minutes in one of my bathrooms before it gets warm. I have written repeated messages in the super's logbook, and have emailed the co-op board repeatedly, to no avail. This week, I sent a certified letter to the building manager. The problem has persisted, on and off, since early 2008. What legal recourse do I have?