From about '91 onwards the venting sytem became more elaborate than just a tube going up the wing. You probably bave what's called a multi-vent tank, 3 vents go into this thin second 'tank' you found to scrub hydrocarbons from the vented air. I'm trying to remeber if it is connected to the carbon cannister under the bonnet which purged by the engine during high-speed cruising or not, the EFi has a connection for it as early as 1990 IIRC.
Anyway, it's an emissions thing.
