I don't know what the differences are between a 1940 radiator filler neck and a 1941 radiator (that's what's on my truck) but the 4pound radiator cap on my truck has two seals like a modern car and functions the same way.

Whether it functions as a return reservoir or not, it can still act as a recovery tank so that excess fluid doesn't dump on the ground. The radiator could be over-filled without wasting the coolant that dumps out.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)