I just don't get it..

I put a radiator in my 67 dart 318,from a 74 dart 318 with ac with coolant recovery.I did not hook up the catch can and just left the hose hang.Why can't I find a cap that will seal and keep my fluids in?I've been to every auto parts store and even a radiator shop,but to no avail no luck.The car does not over heat at all but will always puke out fluid until it gets low and then it of corse it stops why have a psi rating on a cap if you're not holding any psi. Anyone have any advice?