[rotorwrench]

Your set up is still vented although it is restricted. I don't thing the restriction is going to raise the pressure all that much. It would be heat expansion that raises the pressure the most and that would only be after losing all of the expansion space that's left. Once the liquid reaches the restrictor then it could start to build more pressure if the fluid is slow to escape through there.

There should be no problem with that unless steam pressure builds up with too low of a coolant level with resultant overheating. If there is no problems with the engine holding it's mud and the cooling system is working then that should be a functional set up as far as slowing coolant loss.

Skip Haney's 3 PSI check valve would actually hold pressure till it reached the limit but it depends on whether the old radiator can hold the pressure or not. Many of the old model A cars have the Moto-Meters or caps that don't have a very good seal. A good seal would be required to increase the pressure in the system.