Quote:
Why do you suggest connecting vacuum to this valve? Do you know what it does & how it works?
Didn't think so.

The device is called a flow guide valve & it controls the pressure inside the fuel tank & directs excess fuel vapour into the crankcase for temporary storage when the engine is not running.
Don't worry about the engine exploding from the fuel vapour in the crankcase, it's never happened yet with this system.

One line is connected to the fuel tank vapour separator & it has no valve in it so air or vapour can flow in both directions.
The lower one is connected to a fitting on the crankcase ventilation pipe at the back of the block.

The top fitting is connected to a fitting on the underside of the aircleaner.

When the engine is running, fuel is drawn from the tank which must be replaced by air. There is a one way valve in this flow guide valve that opens when the pressure inside the tank drops below a certain value.
Filtered air is drawn down the hose to the top fitting of the flow guide valve [it should have an arrow cast onto it pointing inwards to this guide valve]
This filtered air then flows into the tank & a pressure slightly below atmospheric is maintained.

When the vehicle is stationary, ... in the sun, pressure can build in the tank. When this happens the pressure in the line to the tank opens another one way valve in this wonderous device & the excess pressure, & fumes, are vented into the crankcase for temporary storage.
There should also be an arrow to indicate the flow direction on the lower fitting as well.

When the engine is started, manifold vacuum, via the Positive Crankcase Ventilation valve, draws the stored fumes from the crankcase into the manifold & they are burned before passing out the exhaust.

If vacuum is applied to the top fitting, nothing will happen as the one way valve allows flow into the valve, not out of it.
Lucky this time.

Anyway, we have now determined that this valve controls the pressure in the fuel tank, so your cap should be completely sealed I guess. It might pay to remove this valve & with a hand pump, perhaps a bicycle pump & some tube, test that it does all that it is supposed to do.
It wouldn't surprise me if the fitting with no hose, ... the one that is supposed to be connected to the air filter, is blocked. If it is, then you will end up with serious low pressure in the tank after a while.