You need to set the multiflow valve to go through the filter to check for air infiltration, when the pump has been running a good 1/2 hour. If you get a lot of air from the filter vent, i.e. more than a couple of seconds, then you have an air leak, and check as before with the pump running.

If not then you need to check the pressure side of the pump to the filter. Also you may need a new gauge. Remove it, and if there is pressure from the connection, and the gauge drops back to zero, then the gauge is alright.

You mention a back flow when the pump is shut off. Are there any valves downstream of the filter that might be closed or restricted? That could account for the back flow as the filter depressurerizes back through the pump. This also could account for an air-leak on the suction side, as this air pressure is driving the flow back from the filter.

Hope this helps some more.

Pat.