What I do is pour out the solvent and save it for fire starting fluid. Like brush etc. Never use gasoline, way to high of flash point. Mineral spirits is closer to charcoal lighting fluid. Then rinse the tank and put in fresh. About the only way to get it fresh. I have a tank in the garage and use it once in a while. It is great when you use carb or brake cleaner spray. Lets it run into the tank.
BTW if you let is sit for quite a long time, month or more the heavy parts in the fluid settle to the bottom. And you can pour off the good stuff from the top. My pump does not sit all the way at the bottom so it doesn't matter much to me.