Johnnek, as Doug mentioned, if the car has a stock pressure tank, the neck was originally sized for a European radiator cap which is about 0.060" longer than U.S-made caps. The extra length reduces cap pressure ratings from the cap's built-in spring, so a 16lb U.S cap may leak at only 6-8 lbs of pressure. In addition, the bottoms of the necks are usually quite rough making sealing all but a brand new cap iffy, so the spit-up does not go into the overflow tank.
Fix: take the small tank to any radiator shop and have them remove the OEM neck & solder on a U.S made neck, with a smooth finish at the bottom. I don't recommend using more than 16lb caps as it can encourage overpressurizing the radiator and leaking. Even the best aftermarket street rads are only tested/rated to about 20 psi.
Second chore: when a pressure cap lets go as yours did, the overflow tends to run down the tank, down the inner fender panel and right into the oval lightening hole in the top of the right frame rail. There is no outlet unless you or someone else has drilled the recommended 3 holes per side in the frame rail bottoms. Undrilled rails are usually full of small rocks, dirt and much rust from collected water. I've pulled bolts and broken spark plug ends from some using a magnet. Driving in the rain will fill the rails up to the top with water. Three 1/2" OD holes more-or-less evenly spaced along the rails are all that's needed; some of us also drill 3/8" holes in the extreme bottom legs of the horse-shoe-shaped steel stamping that supports the rear a-arms, for the same reason. The truly paranoid then dump a couple of qts of drain oil in the top holes, with plugs in the drilled holes. Let the oil set for 1/2 day, then drain it: cheap rustproofing!
