@gggGary alluded to a work around/fix in my thread maybe he will expand on it.
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I take no responsibility for this working for you.
Went and looked, sure enough had an unopened K&L kit from the infamous cardboard box of parts that comes with every project bike.
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The suspect
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grab an 18mm-ish socket, stuff the diaphragms inside so the disk contacts the socket edge.
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Gently tap the end of the shaft til the disk takes on a slight dish shape,
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it's aluminum will bend easily. You only need a 1/2 mm or so. This effectively lengthens the shaft causing the o-ring to contact the seat a bit sooner. Check the disk is still at 90 to the shaft, the edge even.
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It will be happy to warp, get all ugly if you overdo the tapping. Done.
IIRC correctly this fix has been in use on Madness' petcocks for ten-ish years.
Carry on!