I cut my teeth on the Renault cooling system. Like many European cars, it was a sealed system with crossflow radiator and thermofan, way before the American manufacturers decided such stuff was good practice.
You had to fill and bleed the system; this can be applied to any cooling setup and will often enhance the result. With the engine at a fast idle and the radiator cap off, you cracked bleeders on the pump bypass and heater hoses until no more air bubbles came out. This may be mimicked by pulling a hose almost right off and just holding it to the hose fitting. Catch the overspill in a bottle or small tub then push the hose firmly home and tighten its clamp. Start furthest from the radiator (rather like bleeding brakes).
When the motor is hot and the thermostat has well opened, you finish the top-up and close the cap. Also marginally overfill the radiator when this is being done; as it purges the excess coolant it also removes air from the overflow line.
The fun thing about the Renaults was most had a serious "issue" with expansion bottles. Early ones were glass, and if the breather vent clogged they could explode (happened to my father back in the day and narrowly missed his eyes with shards of Pyrex glass). Later bottles were translucent plastic, but placed so they got UV degradation and still exploded without much prompting! :roll: That is sort of why I like the old metal ones. A drink bottle is too "poor/starving undergrad" for me.
As I mentioned before, most radiator caps are the recovery type so maybe yours is either blocking the recovery port on the filler neck or just old and not functioning happily... Post back with how it works out!
Regards, Adam.
