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Unfortunately adding shims will raise the IP-just the opposite of what you need. Are there any shims on the spring? Before you open it up that far, give it a LOT of cycles and recheck. If there are no shims on the spring, then try another seat.
A few other things that can lower IP:
Lubricate with a PTFE grease (Krytox, Chritolube, Tribolube, etc.)
Abrade the piston stem where the stem surface rubs on the o-ring.
Remove the flash from the stem o-ring. The flash is the bit of material left over from the molding process. Fashion a cone out of a piece of sandpaper and rotate it inside the o-ring. Blow it clean and re-lube.
Halocline has had success with polyurethane o-rings.
It's the spring centering washer that the MK10+ used. I don't know how necessary it is. Adding any additional shims will raise the IP. A documented issue with converting the MK10 to + is high IP. They recommend lots of PTFE grease, a pretty lame solution if you ask me. You could also try a duro 90 polyurethane o-ring for the piston; those supposedly have better resistance to extrusion and thus will presumably cause less friction on the piston shaft.
Or you can accept the idea that probably the combination of the rounded edge and smaller diameter of the MK10+ piston just didn't work out as well as SP had hoped, and so they went back to the larger size piston in the MK20/25.
You mentioned in an earlier post that you thought using the current MK25 seat is an advantage. To me it's a disadvantage; it's more expensive and harder to find. The rounded piston/cup seat works great on the MK25, but so does the knife edge/conical seat on the MK10.
While using compressed air to remove SP hp seats has always been successful for me in the past, my latest MK5 seat removal was unsuccessful on a really stubborn one. 120psi and it wouldn’t budge. So, I took Zung’s advice and put the seat retainer in the freezer for two hours. Used 120psi again and the seat popped right out. A big thanks and a tip of the hat to you, Zung. The freezer trick really works well.