[shorod]

The radiator needs to be full in order for it to suck coolant out of the reservoir. So if your radiator is not full (possibly due to air pockets that come to the surface (top of the radiator) as the engine cools), the system will lose its ability to create a vacuum and pull from the reservoir.

If you've done work on the cooling system you need to ensure after the engine is up to temperature that the radiator is full. One way we used to do that (before systems had bleed screws) was to fill the system up, leave the cap in the "first detent" position so that it doesn't allow the system to build pressure, get the engine up to temperature, then carefully remove the radiator cap, which shouldn't spray since it's not under pressure. You may be a burp or two. If the level is not full in the radiator, top it off, then increase the engine speed. Usually the level in the radiator will drop as the engine speed is increased. While the throttle is held partially open, again top off the radiator, then put the radiator pressure cap on fully before releasing the throttle. As the engine speed comes back to idle coolant should be pushed in to the reservoir if the system is pretty close to full. It's always a good idea to drive the car several miles and check the radiator level again once it cools.

-Rod