[blucar]

Many people overlook the fact that the average radiator prior to '40-41 does not have a seat in the neck for a pressure cap to work, ergo, a pressure cap is useless.
All of my pre '40 vehicles have had the neck replaced with one that will accept a pressure cap, generally a 7 lb.
The stock expansion tank on vintage radiators is a completely different item than a coolant recovery tank.
If the coolant in a expansion tank gets hot it is expelled and lost out the overflow tube.
A coolant recover system allows the coolant to expand into the recovery tank, when the coolant cools down vacuum within the system pulls the coolant back into the radiator/engine, eliminating air within the system and coolant loss.
Basic chemistry 101, heat expands, cool contracts.