Dishwasher is leaving spots, anyone know how the rinse aid mechanism works?

Please read the entire post before answering, thanks!

My dishwasher sometimes leaves spots on the dishes, glasses, etc.

We try to use the rinse aid but I suspect it's not working right - because when you unscrew the cap to the rinse aid it always seems partially full - it never quite empties even after many cycles.

What I want to know is how does this rinse aid mechanism work anyway? How does it know to dispense the rinse aid liquid only during the final rinse cycle?

I do not need to know how or why the rinse aid liquid works - I would like to know how this screw-in cap actually dispenses the liquid and I can go from there.

All over the internet, nothing mentions HOW the thing works, every post just says how and why you need rinse aid, and why the liquid does what it does. But again, I want to know if the rinse aid container/cap is functioning right.

By the way, I've already tried the vinegar solution, and it still won't dispense the liquid. It always remains mostly full after several cycles - even without adding more it never empties.

BTW its a slightly older Maytag washer, with a mechanical rinse aid container/screw on cap. Nothing appears wrong with it to the eye, it just won't dispense. There are no electrical connections to this cap or dispenser, just a plastic screw on cap with a clear plastic bulb in the middle of it.

Also, the drying selection buttons say "Temp Sense" 'On' or 'Off'. Which is better, I don't have the manual and can't find it online. I assume temp sense means it will shut off after it hits a certian temperature during drying time? I'm asking because sometimes it seems very wet inside after the cycle, like it didn't dry enough either. But if you open the door after the cycle is done it's definitely warm in there, so I know the heating element is at least working to some degree. I get pools of water in cups and things that have flipped back over during the cycle and I have to pour them out, makes a wet mess. I guess this is normal though right? If you put a cup upside down but then it flips back over, your going to get a puddle in it no matter what right?

Thanks and sorry for the long post.