On August 1st I swapped the O-rings on my Hongdian M1 and M2 with thicker O-rings and now just shy of three months later I checked them both. They wrote immediately (no hard starts or skips) and the converters are both still full.
They were already pretty good at sealing but when left that long both would normally have hard started or need some help to flow again.
So I'd highly recommend that change: New o-rings are 8mm OD, 5mm ID, 1.5mm wide.
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