My standard caveat of "I'm not an expert" but...There's a couple of possibilities, there's a yellow O-ring on the hub that's part of the vac-u-suck hub system, and there's an O-ring on the hub itself too. seems like the Yellow one is the likely culprit. I think you can find some guides on it, like
Welcome to guzzle's 4x4 Needle Bearing Maintenance Web Page Even though that's aimed at the needle bearings, it'll still show taking off the hub and where the O-rings are I believe.
I just replaced my hubs with Warn Hubs and blocked off the vacuum lines myself...