What if your pro line sand filter keeps blowing water out between the filter top and the selector valve housing any ideas?

Confused by what part you mean by the filter top and the selector valve housing. If you mean between the filter body and the filter head, it means you need to replace the gasket between the two. Simply remove the filter head and inspect the gasket that sits between the filter and the filter head. If there are any tears or several nicks in it, go to a pool store and just pick up a new one.

If you mean the filter head is leaking, it most likely means the spider gasket is leaking and needs to be replaced