Water Heater

[Ozz]

The seat in the valve may be compromised, it may have a little something under it, I pop them (Lift the lever) then close them and rap on the stem with a screwdriver handle, or small hammer. The spring in the valve may be weak as well.
If all else fails, replace it. It is the safety, and needs to be operational as in, we don't want to have it fail.