Hi please could you help, I have had my bathroom refurbished and had a new power shower mixer valve and electric pump fitted. I also had the cold water tank replaced as it was old and contaminated. I persistently keep get a recurring air lock in the hot water pipe wich I can clear temporarily using the cold water pressure but it always returns meaning the shower will not work as the pump requires a min pressure to operate. This air lock is throughout the pipes and not just common to the shower.

Are you on a pump or city water? By "air lock" do you mean the valve spits and sputters at you or does it make a loud noise when turned on or shut off?
Since air will raise to the top in water have you opened the topmost faucet and attempted to bleed the system? Since the hot and cold lines are separate please explain how you clear a air lock out of the hot line with the cold one. By "cold water tank" would this be a pressure tank on a pump system? Is this cold water tank a galvanized pressure/storage tank or would it be a bladder tank on a pump system. I just don't have enough details to give you a answer. Click on back with more details. Regards, Tom