Fix? Take the cap to the auto parts store. Buy two feet of hose that fits over the cap's ******. Get a hose clamp that fits over the hose. Screw cap on. Slip rubber hose on ******. Attach clamp and tighten. Route hose to hole in fenderwell, or anywhere else to under body (away from manifold.)
Not only will it keep your manifold "dry," it will keep the cap from unscrewing, as it would have to twist the rubber hose to do so. Safe, since it retains the vent in the cap.
~34,000 miles with no problems, after lots of AX and some track time. Good luck.