Are you sure it's an aeration issue? If it's an aeration issue the tank will overfill if the truck uses any hydraulic function long enough. If it only overfills on a lowering of the upright, sounds like you have a surge issue. Did you check to make sure the hydraulics tanks matched exactly? Is the return line the same size- is it pinched or routed so that it collapses internally?

You may have to install a flow limiter on the hoist circuit if all else fails