had a pair of schwalbe ones (the second generation ones not the current model) on one of my road bikes for a while and had my first slow puncture today, the valves do not have a removable core but from memory i did add some sealant many months ago but whatever is inside must be dried up by now.
whats the best thing to do? buy some new valves with removable cores, clean everything off including the bead and start again with a re-seat and fresh jizz?

pop the bead off, add 25ml and re-seat? i just have a track pump but had no issues seating before.

they have been on the bike about 2 years but being busy and having another bike to ride means the tyres are not worn out yet.

Pop the bead off and see how much stans is left, top up as required. I swap tyres on my mtb all the time and have never bothered cleaning off the old gunk.

My LBS had some valves with removable cores in stock so I’ll pop a bead off, fit valves then add some sealant.
Will be very surprised if there is any liquid in there though, it’s been well over a year 😳

Mine have been going for 9 months. For some reason both starting loosing air from around the valve last week. I reckon the block nut had worked loose. Anyway, In opened them up to find gallons of sealant still there. No sign of any drying up or anything. No idea if I have had a puncture or not.

I would buy a MilKit tubeless kit. Fit the valves, add sealant and then check the amount / quality of the sealant periodically using the clever syringe / valve.