[MagicRat]

Your radiator cap is either bad or is unsuitable for this radiator. Almost all rad caps have a spring-loaded drop valve in them. This valve is designed to open when the cooling system achieves negative pressure. This happens usually when engines cool down after use..... usually not when warming up.

In a closed system, the open drop valve will suck up coolant through a hose from the coolant recovery tank. In an open system, it simply sucks air.

If the valve is stuck closed or isn't there at all, the hoses will collapse instead.

As for the coolant loss.... if it really is being lost, the coolant is going somewhere. If its not in any oil and is not leaking, then it's being burned. Do you have LOTS of white smoke out the exhaust?

Sometimes, its possible for someone to not fill/bleed the cooling system properly, leaving air pockets at the top of the engine. As the engine is run, the air works itself to the top of the radiator, so you see what looks like a coolant loss.

Imo fill the radiator, get a new radiator cap that's suitable for the radiator itself and drive it for a day or two. Keep an eye on the coolant level and see it it drops any more.