No they're not one use, I open mine when I empty the tank, empties a lot faster. Like Yvonn said, after its cool open a hot water faucet to relieve the pressure on the tank. Open and close the valve several times with the lever on top letting it slam shut with the spring, hopefully it will knock off any crud on the seat. Pressure the system again and see if the leak stopped.
If you still have a leak the seat is probably bad, carefully unscrew the valve from the tank, go to a hardware store and get a replacement. Put pipe dope on the threads and screw the valve in until its tight, check again, but you should be good to go.
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