I installed the replacement cylinders over the weekend. I did remove the seals and pistons, coat everything with red rubber grease and replace.
It seems that when the cylinders are new, they have no corrosion protection behind the outer seals. All they have is a thin layer of whatever oil it is they use for assembly.
Unfortunately I didn't manage to bleed all of the air out of the system, so will have to have another go at that next weekend.