What am I missing. Trying to seal a WTB Cross Boss. Rims are Pacento CL 25s which previously held Schwalbe S ones perfectly. One CB went up and stayed. One won’t hold air overnight but poops onto the bead easily and is so tight its a bloody nightmare to unseat. Its got yo be a valve surely. No bubbles around valve with 70 psi in yet later its flat. Stans valves. Gorilla tape which was perfect so reused but then replaced as an attempt to seal. Tried with and without sealant.
Grrrrr

The rim is box section, and the valve seats onto the inner rim face, and if it’s leaking there, the air/sealant will mostly flow into the hollow section, so I’m not sure if you can feasibly test the valve seat for leaks.

Make sure the valve stem lock nut is done up really tight. And make sure the valve core is inserted properly (don’t over-tighten it)

I don’t understand your comment about tyre seating. You say it pops on easily, but is a nightmare to get off ? How can that be the case ?

Have you pumped up and squirted soapy water all the way round the tyre bead and sidewalls ?

Put a foot of water in the bath & dump the whole shebang in there – a bit of old school leak searching?? 🙂

It doesn’t take much of the traditional grief to get it to seat, no soapy water or anything, just a few fast pumps. Pop, pop, pop, seated and inflated. The thing is so well seated that I can’t break the bead to get levers in. On my enduro bike I just boot the bead in but that’s a 140 x18 knobbly.
Anyway after posting I did bung it in the bath. Wife wasn’t happy when I asked her to budge up and hold it between her knees 😛 but that showed bubbles from every spoke hole bar a few.
So its either re tape or check the valve is tight but I already dog it up with pliers.
Warm tyres do pop off easier though. Or give in and use a rim strip.

When I do mine I pump them to about 20psi then spend a good 5 mins swilling the tyre round to spread the sealant into everything then pump to about 60psi and leave overnight