It is correct to check the Jet pump out , but have you cleaned out the strainer - filter Part No 141787 ?. This is mounted on the right hand side of the bell housing , straining the clutch oil on its way back to the hydraulic tank. You should disconnect the hoses and remove it from the machine for cleaning. Not the easiest of jobs , but it would be restricting the flow back to tank.
we have a forklift with the same problem. what did you do to fix yours?
Not familiar with your exact model but the wet bell-housings I've serviced sometimes clog their drains, leading to an overfull condition. Have you checked yours?
On the smaller trucks with the wet clutch, you could slide the oil tube to lower the output. I am not sure if the larger trucks had this feature.