It’s a non-issue because it didn’t cause the overflow. In an otherwise non-plugged system it is irrelevent. Additonally, unless the landlord was informed it was present landlord is not likable for damages caused because landlord is not negligent for failing to repair the defect.
Given this statement:
landlord may not be negligent because nobody was aware the valve stuck.However, the tenant claims that he never noticed it to be faulty and never noticed that water would continue to run into the toilet.

