If you don't get any response in here, it's probably because people won't be familiar with the "mixing valve" and "air in the lines" you're referring to.
The only place where I know a mixing valve is used in some houses is to mix both hot and cold water to supply warm water to the toilet tank. That's done to prevent condensation on the toilet tank. Is that what your mixing valve is used for? If not, can you let us know what it is, where it is and what it does. So far as I know, there's no water mixing valves used on hot water heating systems.
Air in your water pipes is only a concern if you're talking about a hot water heating system. That is, the heating system that heats your house in winter. If you heat water meant for cooking, cleaning and bathing in a water heater, then you're ALWAYS going to get bubbles of oxygen gas collecting in the hot water supply piping to your faucets, but those oxygen gas bubbles do no harm whatsoever. Cold water can contain more oxygen gas in solution than hot water, so as you heat cold water in your water heater, oxygen gas can be driven out of solution and will collect as bubbles at the top of your water heater or in your hot water supply piping. If you turn on your hot water faucet, you will often see/hear a small blast of gas coming out of the spout with the water. The gas coming out is oxygen gas that was driven out of solution from the cold water when it was heated in the water heater. You simply run your hot water faucet to remove it from your hot water supply piping.
So, we're not sure if you're talking about domestic water supply piping or a hot water heating system. A "mixing valve" could also mean a tub and shower faucet cuz no one showers in pure hot or cold water, and those things do have anti-scald mechanisms on them that do have adjustment screws. Suffice it to say that we're not really sure of what you're referring to by "mixing valve" and "air in the lines", and until we're sure, we can only guess at the problem.
Could you please clarify. Thanks.