Although the landlord's responsibilities may vary based upon the terms of the lease, ordinarily the landlord of a residential unit is responsible for maintaining the plumbing. If you have not done so already, you may want to send a certified letter to the landlord demanding repairs. You may also want to consider retaining an attorney to represent you.
There is a implied warranty of habitability in the lease. In other words, your land lord has the obligation to provide you with a livable apartment. To protect your legal rights you may want to consider putting your landlord on notice of his breach pursuant to the terms of the lease.
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