I have a bathroom tub facet does not seal well. The facet that fills the tub has a plunger on it. When the plunger is lifted it turns on the shower. This plunger doesn't seal tightly and a lot of water leaks out of the tub facet (at the opening as if the plunger were down), which greatly reduces the shower water pressure. What is the best way to fix this? The shower fixture appear to be relatively new so I would prefer not to replace the whole set. I have access to the plumbing through an access panel.