Salt water generators use salt (like the pellets you put in your water softener) to generate Chlorine. They're a separate system that is required to be installed into your plumbing.
Instead of hauling chlorine, you'll be hauling salt (not as much mind you).
It just seems weird you'd be losing prime when replacing the filter since it's on the pressure side and at worst you'd see water leaking out somewhere, so your leak or issue really should be located on the suction side.
When you run your pool without the vac attached - everything works as it should?
Have you cleaned and lubed the gasket on your pum lid?
Have you verified any quick disconnects like at the skimmer or pump are tight and also lubed?

I find most issues arise when owners reconnect their equipment in the spring and don't do the proper maintenance on the equipment prior to install, such as cleaning off dust and debris from connections, lubing gaskets, tightening the drain plug in the pump properly, reapplying new teflon tape to any screw in connections...etc
Also, have you looked your vaccum over to see if there are any obstructions in it, sometimes, an obstructed vac head will create so much suction side vaccum in teh lines to force the system to draw air through a minor joint leak normally not noticeable.