The coolant wont return with the pressure release valve rad cap but will with out it?

The car had a 13 psi cap when I got it and it never had coolant come out but I put some conditioner in it that made the fluid level come up too high in the radiator so that is why it was coming out I believe.

 

+1 jlg - its called a recovery cap. they usually look similar but have a 2 way seal in the cap to allow coolant both out as it pressurises and then back in as your car cools. you still use 13lb

 

All radiator caps made today will work with a coolant overflow bottle, even the lever type.
The lever type allows you to release any pressure in the system without taking the cap off. It keeps stupid people from scalding them self’s with boiling hot coolant.
If you look at the bottom of the cap you will see a small silver disc, that is a one way valve. When the pressure in the radiator gets to the pressure listed on the cap, it pushes the rubber portion of the cap off the neck, and allows the coolant to flow into the bottle. After you shut the engine off, as the coolant cools off, it contracts, and causes vacuum to form in your radiator, this pulls the coolant from the overflow bottle back into the radiator through the small silver valve.