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

Now that you have fit the new tank are you sure that the take up hose fittings are secure dose the pump sound like it is cavitating It's usually case that air is being drawn into the system some where, dose the hydraulic filter have a good seal has the take up hose been covered in battery acid it might have perished somewhere not enough to leak but enough to draw air into the system, I hope my random thoughts give you some ideas where to start looking.


Regards Titus