When you open the air release on the filter, what should happen is that it should vent air until the filter is full of water and then it should vent water.
If air gets into the system after you vent the filter, then if you open the vent again, more air will vent until the filter is full of water again.
Since the water level in the filter isn't rising enough to fill the filter with the pump running, the pump isn't pumping the amount it should. If it was all an air leak, there'd be a lot of air coming out of the returns.
Have you looked at the inlet side of the impeller to see if it's plugged?
If air gets into the system after you vent the filter, then if you open the vent again, more air will vent until the filter is full of water again.
Since the water level in the filter isn't rising enough to fill the filter with the pump running, the pump isn't pumping the amount it should. If it was all an air leak, there'd be a lot of air coming out of the returns.
Have you looked at the inlet side of the impeller to see if it's plugged?