..want to install a rad over flow system in my Duster.
..w/15 lb cap,system is pressurized until 15 lb is exceeded,cap opens and excess flows to reservoir...cap closes.
..how does it get back into rad if cap is closed...is this also a check valve for reverse flow when cool?
..what cap do i need for this, part # ???
..which newer tank adapts easily?

Yes, there is a one-way valve on the bottom of the cap--look for the metal dime-sized 'button'. You can pull it down with very little force, the force that does that is vacuum in the cooling system as it cools and contracts.
Very simple, very effective

As far as tanks you NEED on that draws from the bottom--either by the hose connecting to the bottom of the tank or a tube from the top cap that extends down to the bottom of the tank

as system cools, a vacuum is created,pulling coolant back in.. any bottle will work, as long as the hose is at the bottom and the top is vented. any standard cap will work. you are making it more complicated than need be. you can get a nice aluminum tank, or slide a hose into a half full antifreeze jug. its that simple

..tx guys
..can't help but think i'm leaving some cooling capacity on the table with that top tank always low.
..now to find something that's a good fit.

Way easy to do. We are putting a recovery system in a challenger this week. Guy hates the anitfreeze puddles it leaves and wants to cleaner with it.