Their are 3 things that will make a T&P valve leak. 1. if the Tempature is too high (easy to check, run hot water, if it is steaming and bubbling it is too hot) 2. if the Preasure is too high (or sometimes too high) it will leak. (if no. 3 doesn’t fix it, have a plumber check no. 2) 3. the P&T relief valve has gone bad and needs to be replaced.