I use about 2 wraps of teflon tape....making sure not to get it past the first 2 threads so none ends up in the system...then a screw it on a half turn and drop a drop of Locktite thread sealant on it...that's the best i've been able to find. My truck sat for almost 2 months after Tex Mex with 200 PSI in the tank.....over 2 months it dropped to around 197. Been sitting at ride height in my shop for the last week....and hasn't moved a bit. I've never had very good luck with anything else. I would always have to redo alot of fittings...since I started using this I havent redone one yet.
once i had this problem, if you put a fitting on the end of the dump with about 2-3 inchs of airline coming off of the that it will work, it may have something to do with back pressure