Thanx guys for the great feedback.
I really appreciate this.

I asked my servicemen to proceed with the coolers renovation.

@grubbymits
We checked very thoroughly, there's no pipe between rocker cover and manifold.
We have several hundreds of trucks under our maintenance, Linde, Still, Toyotaa, Yale and simply each brand and type has the vent for deaeration.
This Hyster with Yanmar is the first truck, where we can't find it.

Recently we found, that similar engine is installed on some Doosan 3,5 tonners and in the catalogue there is something that looks like round cap on the rocker cover. Perhaps this is presumed for installation of the vent.
Unfortunately nothing like that we found on this Hyster.

I am still stupid with this engine.

Regards to all.