If the valve was working, and now isn't, it's usually time to replace it. Some of them have rebuild kits, but it's a pain if that doesn't fix it, your labor, time, and money now is all wasted since you have to replace it anyways. Just so we're on the same sheet, this is a prv for a city/town water system on your main input, and not a pump control, right?
If your WH's safety valve, the T&P is leaking water, you may also have a bad expansion tank, and that may need to be replaced. When you heat the water, it expands, and needs somewhere to go which is the expansion tank.