i don't view fluctuating pressures as normal. in the case of air, the discharge pressure is going to fluctuate, possibly wildly. the overall discharge pressure will be abnormally high. air and water are considered to be 'non-condensables' in a refrigerant system. one way air can get in is if there is a leak on the low pressure side of the system that causes the system to drop into a vacuum while running. the air sucks in through the leak. if you called the tech because it wasn't cooling and he saw it was low and added refrigerant, then saw the fluctuation, that makes sense. if this is a new problem, that is the only explanation. if this is the case, the tech needs to leak check your system, seal the leak, replace the liquid line filter drier, evacuate and recharge. removing the refrigerant with the air in it is the correct procedure, but make sure the leak is identified and dealt with. otherwise, you will have a repeat performance.