Assuming the level in the reservoir has been adequate, about the only way air can get into the system is through a leak on the suction side of the pump. I'm not familiar with that model, so this is mostly a guess. Is there a filter,? If so, was it recently changed? The filter change can introduce a bunch of air that needs to be bled out. People often end up with a leak at or near the filter after it has been changed. Is there any hard line in the suction connection from the reservoir to the relevant pump? Sometimes those lines crack (often from vibration). Is part of the suction connection a hose? If so, check it for deterioration.
