When I finally got around to firing up my new air compressor, the drain valve wouldn't open. You're supposed to be able to open it with just your fingers, but I couldn't get it open even with a pair of pliers and destroyed it in the process. And, of course, no receipt, no warranty. Someone suggested that instead of just replacing the valve, I should put the new one on an extension so that it would be easier to get to than it had been at the original location on the bottom center of the tank. So, after a trip to the home improvement store to get all of the parts, including a new drain valve, I was ready to repair it for less than what the manufacturer wanted for a replacement valve.
There wasn't much to it. Take out the old valve with a deep socket. Use Teflon tape on the new parts, making sure that the tightening of the pieces tightens the tape, too. And don't over-tighten. The new pressure relief valve has a wrench fitting, but the instructions say finger-tighten only. Make sure that the drain valve is protected by the wheels or the tank so that it doesn't get hit accidentally.