I would never drain my water heater, especially if it is old. First of all, there will be sediment caked onto the entire bottom of the tank, which will will not be removed by just opening the very small drain valve. Second, if you open the drain valve and then close it, there is a good chance that the sediment will clog the inside of the valve and cause it to leak. And third, the Rheem owners manual does not recommend ever draining the water heater. It only "suggests" that you drain a small amount of water from the tank every month or so, which I would not do either.
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