I have a 3/4 inch check valve on my hot water heater. It has a threaded top and a small side screw. My question is, do these go bad? What is the small screw for? What is the top access bolt for? I am getting hot water back flow through my cold water pipes. I can't get the top ov the valve off to see if it is working properly but if I take the small screw out no water leaks out. Thanks

The big nut on top is for you to replace a "gate" inside, this is what makes it a check valve, the gate seats against a brass seat.

The nut on the side is to unscrew the shaft that the gate swings on. I am not sure that you could find anywhere to get the parts that you need, except the manafacture. You might find them on the internet.

I bet you it will be much cheaper, and faster, to replace the check valve rather than look for replacement parts. I have never even seen replacement parts in store offered for sale.