Hi, concerning the air in the hydraulics.
Usually, Linde trucks deaerate themselves while working. If the air remains in the system, it means it's getting sucked.
Did you replace the suction filter (in the hydraulics tank)?
If yes, probably you missed to change the big O-ring under the cap.
I've seen it on many trucks, that after the filter replacement, the system suc.s the air under the hydraulics tank cap.
Check the cap itself. It may be damaged. Check the other hydraulics filters.
The oil replacement procedure is described with details in operators instruction. The drain plug is under the truck, on the left side.
You pour the oil through the filler (where the vent filter is).
Concerning the yellow light (does the symbol looks like a spiral or spring?) probably IFD45 was right. It may be glow plug or some engine problerm