Sealant won't stick to the metal part of valve stems I found so they don't wanna seal up. I had problems on a set of superstar carbon rims as the rim bed is a stupid shape making it impossible for the valve to sit low enough to seal up. I ended up fitting the valve then applying puncture repair glue around the valve Base and rim to fill in thegaps and give the Sealant something it can take to. I also think Stans rim tape is the worst thing to use. its to rigid and not sticky enough so doesn't want to stick on rim beds with any sort of contours or square/lipped edges. Gorilla tape or elec insulation tape done tight works far better. That tool looks like summat from the missis toy box. I just use my thumb on the rim. Gets the job done ;-)
the american classic valve stem came with 2 grommets.
a big one..... that i put back on the base after i filed the aluminum ..
plus an additional much smaller grommet . that i placed on the valve stem where it exits the rim.
then i placed that plastic contoured part on,
then , the red anodized presta nut.
i do believe the other various valve stem bases which are made of rubber. are the best solution for tubeless set ups. .
As someone else said that bit you put on the outside should be on the inside of the rim & as others said the Stans valves look a lot simpler in fact fitting them I've never given any thought to, put through hole tighten job done super easy.
My (most?) carbon rims have an inner hoop where the spoke nuts connect and an outer hoop with holes to access the spoke nuts. Taping these rims means taping over some pretty big holes and creating a mostly-carbon "inner tube" closer to the center of the wheel. See https://www.pinkbike.com/photo/14851730/
So, if air is leaking into this "inner tube", it's likely the easiest place for it to get out is at the stem. Don't break your rim/stem by overtightening, find the tape leak and repair or just retape completely
I have been using gorilla tape on the rim for a few years, and I also used to have the chronic leaky valve issue. As someone else mentioned the air could be from poor rim tape, but quite often the hole in the rim for the valve is just too big, and pumping the tire up just makes it worse. For some time now I have been wrapping the valve stem in tape as well. Just enough to make it snug in the rim hole, and not so tall that the nut will not go on. May take a minute to gauge all that but then the sealant seems more likely to clog around the valve and you also will not upset the seal when pumping the tire up with a stupid little mini pump. ( we all do that from time to time).
I had the same exact issue. I used gorilla tape, but make sure the width of the tape covers the entire internal section of the rim, not just the holes in the middle. That worked for me.
It could be water in the rim, not in the tire but in the hollow section.
Yeah, I’ve had this happen from riding through streams, etc. Then later had issues with water getting under the tubeless re-taping and messing it up if you don’t shake all the water out first and let it dry some. Had to learn the hard way.