car - 1.8 2006 Mk 2 ford focus sport TDCI.

think it has air lock.

Fill expansion tank and drive a short while, half hour later the bottle appears empty yet It does not overheat and still blows hot air and when removing cap the water rushes up and fills bottle then gurgles back down.
I keep water in car in case of emergency but despite the bottle showing empty I can drive for weeks with hot air still blowing inside and no overheating but I decide to just top it up anyway after a while, but as said as soon as I top bottle up it shows empty 30 mins or so later yet can still drive it for ages / weeks without problems.

how would I bleed the air out if it has any trapped in the cooling system ? it has no bleed things as far as I am aware.

What if I take cap off and drive but not let ti get to low and keep topping it up (without bottle getting empty)