I think the tank vents through the charcoal canister, you could also have a vacuum leak in the canister, try pinching off the vacuum line on the canister see if that helps, also your canister filter could be caked with mud and debris.
86 CJ7 5.7 vortec. 4L60,D44's,OBA,Air tank bumpers w/D rings detroits,4.88,35" bfg, mt's 9500ti warn
86 CJ7 2.5 detroits, OBA 4.56, air tank bumpers, w/D rings,9500I warn 31x10.5x15, Trxus
86 CJ7 bone stock, w/factory 44 never seen the dirt soon to be for sale
87 Wrangler also never seen the dirt.