I have a 1983 Linde H70D. Drive was starting to get a little sluggish. Then it slowly stopped driving. I have replaced the main hydraulic oil filter in the tank (with new o rings on the aluminium suction housing). I have also replaced the old oil suction hoses. I don't think it is a drive coupling problem as you get air bubbles coming from the main hydro pump after the ruck stops.

The mast functions are all fine with no signs of cavitation.

Is there an air bleed procedure?

What is the purpose of the small hydraulic header tank located near the battery? This was bubbling air before drive stopped). It also bubbles air after the truck is now stopped.

I assume the charge pump for the main drive pump unit is the one located at the back of the engine?