18V high compression are slightly higher at 9.0:1 and 170psi at 275rpm.
The port on this valve is connected to a port on the front tappet chest cover, which contains a wire gauze mesh that acts as an oil-trap as well as a flame-trap.
The oil filler cap was changed to the vented type, which lets fresh air into the crankcase via a wire gauze filter and a restriction.
The effect from either is minimal, which way any air would flow is a matter of conjecture, and if there is more suction on the rocker cover hose it will be pulling unfiltered and potentially moisture-laden air into the crankcase from the timing cover hose - not ideal.
At least any fumes pulled out of the crankcase in that direction would be burnt in the engine, if the flow is the other way they are just pumped out into the atmosphere.
Emptying the carbs would prevent that, but if you do the wet test some oil is likely to get in anyway. On the pukka kit there are two pressure gauges, one either side of a restriction, with the 'inlet' gauge connected to a compressor via a control valve, and the 'outlet' gauge connected to the cylinder. put it in 4th gear with the handbrake firmly applied to prevent the compressed air pushing the piston down.