How can air get into the cooling system?

[TVR Beaver]

Two post now on checking the small pipe.. have you checked it ?...

[Alexc1984]

I reckon it could be happening when the car is sat with a hot engine after switched off, with the coolant circulation off and the fans off. Some form of leak that only takes place when the engine is hot, due to some expansion. That could stop the hoses from holding pressure to prevent the coolant from boiling. Vapourisation could then produce a steam that is condensed when the car is cold, pulling a vacuum and sucking air in which displaces the water somewhere to force it into the expansion tank. Have you checked the swirl pot cap is tight and holding (ptfe or copper gasket). I assume the swirl pot level drops as a result of the expansion tank rising?

[jamienshelly]

Check your coolant level when cold at the swirl pot with the pressure cap ON, if the coolant is at the top then you should be ok (assuming you have been running the engine hot and cold, and with the heater valve open). As long as there is coolant in the expansion tank you should be ok.
If the level at the swirl pot is low, top up and open the radiator bleed point and check for air, then refill swirl pot and recheck after the engine has got to temp and cooled again.
Ideally if you suspect a leak then get a pressure test done

[The Aardvark]

TVR Beaver said:

Two post now on checking the small pipe.. have you checked it ?...

I haven't, but Its currently back with mechanic for further investigation.

Thanks to all so far for their comments - I'll post up when we (hopefully) get a definite answer as to what's going on here.