hi
dont want to be critical of you statement above but the exspansion tank has air in it all the time with or without removing the cap and lets face it if he removes the cap after driving it about he may get burnt when the water bubles out

what you do is fill the system to correct level loosen the pipe from the top of thermostat housing the small pipe on top and then run the engine to warm it up so the thermostat opens but in this case he has no thermostat so it can be done from cold in this case, once warm remove the pipe whilst engine running and hold it higher than exspansion tank and as the water pump circulates it will pump the air out if any but dont let the tank run down to low
once the water pumps clearly with no spurting, pop the pipe back on and tighten and refill the exspansion tank with hot boiled water never put cold water in a hot engine and this will sort the air in the system out if its trapped
mark