A lot of times the mfgr, (or a dealer repair) drills a hole and "Pins" the gland nut in place with a screw, pop rivet or some other pin ...at the seam between the gland nut and the cylinder body so the glad nut doesn't loosen itself. It's very effective. Over years of use, the head of the screw, nail or rivet, whatever they used to 'pin' the gland, it gets sheared off from rocks, etc and is very difficult to see. A lot of guys don't see the 'pin' and therefore cannot get the gland nut off and cause a great amt of damage trying. An aluminum pop rivet will probably shear with a big wrench, but a metal screw or nail won't. A cleaning and careful inspection can usually ID the 'pin' and drill it out with a small bit. Then the gland usually comes off fairly easily. Something to keep in mind for the next time. All mine were "Pinned" from the factory. When I re-did my seals, I purposely re-drilled and installed a new screw (the 'Pin') on the backside of the cylinder away from damaging objects.
I didn't replace the Piston on my cylinders. In the future I would definitely do that. The new seals stopped the leak out of the cylinders, but there is still a slight amt of 'leakdown' of the cylinders.