Truckster (Vic) replied:

Start car, run it for 20 secs with cap off, fill, leave running until themostat opens, and fill, put cap back on. turn off

zigglemeister replied:

The radiator itself should be completely full. If you have a plastic overflow bottle that should have a breathing space - should have max-min lines on it somewhere - but the radiator should be full. The radiator will "breathe", if necessary, by causing the radiator cap to lift up a bit and let some water out, if the pressure reaches that point. If you have an overflow bottle, the excess water goes in there, if you don't, it just runs onto the ground. Either way, as long as your radiator cap is in good nick, the cooling system can't become over-pressurised.

Simon replied:

If you have the overflow bottle radiator should be full, if not, as long as the water covers the core in the top tank.
Dont keep filling a radiator without an overflow tank cos it will just blow out the excess and find its own level anyway.
If it is regularly down below the core start lookn for leaks.

Mick replied:

The radiator should be full. All vehicles have an overflow bottle and rises and falls occur in that.

Brian replied:

Heres a little aside to the question. Check manufacturer specs on what coolant, if any, should be used in the radiator and USE IT!! I had one of the first series water-cooled VW Transporter vans and because it didn't have the correct coolant, the motor corroded from the inside out!!! Many thousand dollar repair!!!!!!!!!!!! There are other cars that this applies to as well. So when I bought my GQ, the first thing i did was have my mechanic change ALL the oils and make sure the right coolant is in the radiator!!! Good advice people, can save you $$$$$$$$'s!