The cap is 13 PSI for the 300-6. If the gasket of the cap has cracks, replace it. If it doesn't, have it pressure tested, replace if out of specs.

If the Radiator is loosing fins, and the drain hole was plugged up beforehand, I'd think about getting a new Radiator.

 

what would be tha best way to get rid of those air bubbles?

 

I've always been told to let the truck run with the radiator cap off, let it heat up, you should see the bubble come to the top.
Don't take my word for it, I've never done it.

 

That will work, and that's what I do if a air bubble is suspected. However be aware the air bubble(s) may force some of the coolant out of the Radiator before it "burps". Let the radiator "burp", then add coolant. Be sure to keep an eye on the temp gauge as well cause it will tend to overheat with the cap off.

 

Well the years of wisdom from my Uncle have paid off haha. Now I know how to get air bubbles out of the coolant system
Good luck Napolean