Yes, air can get trapped in proportioning valves. This is because the valve contains a small chamber filled with air and gas which can become blocked or restricted when pressure builds up within the system. When this happens, the air pressure may not be able to escape through normal means and gets stuck in the valve. The result of this is that it causes a restriction on flow within the system as well as an increase in pressure which could severely damage other components if left unchecked. To prevent this from happening, special techniques must be employed like using high-pressure relief valves or vacuum breakers to ensure that all air pockets are eliminated before they cause any problems. Additionally, regular maintenance should also be performed on these components to make sure no debris has built up around them blocking airflow or causing further issues.