1. On many Panteras, the swirl tank cap neck is poorly soldered from the factory, leaving lumps at the bottom internal joint- right where the cap seals. A fresh, flexible rubber on a new cap may seal OK but soon begins to leak with age.
2. 20 psi is too high a pressure rating cap and can wreck a street radiator by "ballooning" the core. Those are for NASCAR with a welded aluminum radiator! 16 psi is all the pressure most street rads are rated for- if that much. Genuine Euro caps will be marked "1.1-bar" which converts to about 15.8 psi U.S.
3. On stock swirl tanks, the height of the cap neck fitting is for European caps which are a little longer than U.S. caps. To fix both these factory screw-ups, take your stock tank to any radiator shop and have them desolder the tank neck and add a new U.S. made tank neck. Bingo- no cap leaks. I've also seen cracked overflow hose spigots which leak in the same area. This also causes no water expansion into the big overflow tank during running..
4. Incidentally, swirl tank cap leaks will run down the swirl tank, over the mounting bracket and inner fender, directly into an oval frame hole! This often results in loss of coolant with no tell-tale spots on the garage floor. But of course, the frame is sealed EXCEPT where the top hole is, so the coolant sits there and slowly rusts out your frame! All the above is NOT speculation it has happened to quite a few stock Panteras.
5. The secondary fix is to drill three 3/8" holes spaced along the BOTTOM of the frame rails, on each side. Splashed up rainwater will also fill the frame rails. I've seen frames full to the brim in wet weather! Be prepared for a cascade of rusty water, fine rust, sand and god-knows-what else from your fresh drill holes! A long flexible magnet down the oval top hole may find larger debris; I found a broken spark plug inside our car's frame! Such finds may solve an elusive rattling noise....