Unusual Air Lock Solution

Neighbour asked me yesterday to have a look at a air lock problem he had with his upstairs cold water supply to his toilet cistern and the cold tap on his mixer tap, both upstairs. His CWST in the attic had run dry about 2 weeks ago, the hot supply was perfect as was the cold supply to his (pumped) Triton 90 SR shower.
The CWST has two outlets, one directly to his HW cylinder, the other to the toilet cistern and the mixing tap, the shower 1/2" supply is teed off this outlet just outside the CWST.
I disconnected both cold water supplies to the cistern and the mixer but this didn't clear the air so I re connected the cold mixer suppy and blocked the mixer outlet with my thumb and gradually moved the mixer from fully hot to sort of mid position until I felt the cold supply getting hot, eventually, after approx. 10 minutes everything returned to normal. What surprised me is that the hot supply which is fed from the CWST (and the HW cylinder) is at exactly the same head as the cold water supply was able to displace the air or whatever, I thought I might have to rig up the mains supply to clear the air lock.