Hm,...Vivien, what you describe has also happened to me with my Sailor carts.
Although many folk here do not advocate the squeezing of cartridges, Sailor in fact recommends that you squeeze their cartridges. I believe this instruction is on the pamphlet that comes with the pens.
When I plug in the Sailor cart, I point the pen to the ceiling & squeeze a few times till the ink starts to show in the feed's combs. Then, I let this ink drain back into the pen.
At times the cartridge may need an additional squeeze when it's half empty.
I've had no ink hanging up inside either the blue/black or the Kiwa-Guro black cartridges using this squeezing method. The downside may be that squeezing the cartridges may make them less reliable, if you wish to refill them later.
Mind you, I never refill carts,..they are only used for travel.
As a matter of course I install one of the little glass/plastic balls that are found in the international carts into all my international converters. This works very well.
I've also tried to put the same little ball into the Sailor converters, but it did not seem to fit very well.
I suppose you could try inserting a small glass bead into your Sailor cartridge once the nipple has pierced the cartridge's seal. The cart has to be pulled out of the pen, the bead inserted, then the cartridge plugged back in the pen again.
May turn out to be a messy affair.
To the OP:
I did not mean to derail this long & fascinating thread.
Just wanted to help a fellow FPN member using a Sailor Sapporo Black Currant.
*Sailor 1911S, Black/gold, 14c. 0.8 mm. stub(JM) *1911S blue "Colours", 14c. H-B "M" BLS (PB)
*2 Sailor 1911-M Burgundy/gold pens: 14c. 0.6 mm. "round-nosed" CI (MM) & 1.1 mm. CI (JM)
*Sailor Pro-Gear Slim Spec. Ed. "Fire",14c. (factory) "H-B"
*2 Kaweco SPECIAL fountain pens: 14c."M" "B",-0.5 mm & 0.7 mm stubs (PB)
*Kaweco Stainless Steel Lilliput, 14c "B" -0.6 mm. stub (PB)
*Montblanc 254, 14c. "BB" (1.1 mm?) flügelfeder factory stub