I have a question about my shower valve. The valve in my shower is sticking. It's the type that you pull the knob to start the water flow. Pulling the knob is a little tough to do but pushing the knob back in to stop the flow is really hard. I have to twist and push hard to get it back in. Is there any way to fix this by lubricating or does the valve have to be replaced. I'm in the process of selling my house so if there's a quick fix that would be much better.