Not familiar with the Yanmar engine, but the Hyster's I work on have the GM engine. They have a vent from the valve cover to the intake hose, between the air cleaner & intake.

When you say air pressure, I assume you mean built up pressure in the engine's crankcase? If so, how many hours are on the machines? What grade of engine oil is being used?

Even with built up crankcase pressure I'm not sure this is the cause of your overheating. I've had to install vents in the cover over the radiator to cool off the ones I work on. My units weren't overheating, but the electronics & LPG regulator were constantly failing due to the heat. Now that the vent is in place, the heat related failures have gone away.