Selgas
Simply because the TPR valve is letting a little water pass through it does not mean it has to be replaced. It is designed to allow a discharge of water to pass through expansion when the water heater is recovering (heating up). By installing a cold water reief valve on the inlet line to the water heater you can minimise the discharge of hot water by making the system discharge cold water instead. The cold water TPR vlave would want to have a maximum setting of 70% of that of the hot water one for it to work properly. 
