Buffalo radiator mystery - J model

[MikeH1A]

I've just emptied the radiator and then gave it a bit of a flush until the fluid coming out was clear. Replaced the bung and then set about filling the thing up again. The plan was 50/50 of distilled water and coolant (for aluminium systems). According to the service manual the old girl takes a total of 4.5 litres. However ... I've put 3.5 litres in and the fluid is sitting right up by where the cap goes on. Whilst filling her up, the fluids slipped down nicely till about 2.5 litres went in and then I had to pour a small bit and wait for it to drain down and repeat this - was a slow process.

Anyhow, I've still theoretically got 1 litre to put in and no space to do it. Any ideas what's going on here ... or what I've done/not done?

Thanks for any advice.

[Coyote]

Run the motor for 10+ minutes to be sure the thermostat has opened. It will probably take some more after that, but not all of it. It's kinda like an oil drain. Some always remains traped in pockets and recesses and in the overflow tank. Don't forget to add stop leak to your coolant solution. The correct amount is 70cc.

[MikeH1A]

Thanks for the advice. I shall try that this weekend. Appreciate the reminder about the stop leak additive.
Mike