There are a few problems with plastic valves,
1) plastic O-ring
2) plastic seal
3) cracked body
4) small opening
They really seem to built to be as cheap as possible
The plastic seals are a pain, they distort, stretch, become brittle, break and seal poorly, I tried replacing them with quality O-rings but they still have a problem with grit
The small opening only lets water out slowly, meaning they don't flush sediment very well
The solution is to replace it with a 3/4" brass ball valve. This opens quickly, has a straight through, unobstructed opening and flushes sediment superbly. It only cost $11 and the ball works well with grit since the seal wipes on the ball. Pretty much guaranteed not to leak
Steps
1)Turn off gas (or electricity)
2) shut off water
3) remove the old valve, This one had to be unscrewed 8 turns which disengages the coarse thread, pull the valve out 1/8 inch which engages the smaller retaining thread, and then turn it anticlockwise to unscrew right out
4) wrap Teflon tape around the 3/4" iron nipple from the tank
5) screw on the ball valve (in open position
6) shut off the valve, turn the gas on and light the pilot - or switch the electricity on,
Of course while you're doing this hot water is draining from the tank. Wear gloves to avoid scalding and make sure the water drains into the floor drain. If it doesn't you'll have to attach a hose and drain the tank completely. By doing it quickly I lost only about a quarter of the tank of hot water.
Please note I am referring to the drain valve NOT the pressure relief valve. The pressure relief safety valve) at the top of the tank) must only be replaced with another of the same rating
