ok it's the mast cylinder
let me ask you this
is it leaking oil around the top of the cylinder where the ram comes out?
or, is it drifting down when raised up?
IF it is just leaking oil out of the top then you may can get by just putting new seals in the cap where the ram comes out.
But IF it is drifting down then there might be 2 things causing that, 1-the packing on the ram OR 2-the safety valve could be defective, all cylinders on masts have one, usually they are located inside the base of the cylinder or in some cases they are part of the inlet fitting depending on the design. The purpose is to keep the cylinder from dropping too quickly if a hose were to burst. Some call these velocity fuses.
From the sounds of it you already have the cylinder apart so changing the seals on the ram still may need to be done but if they aren't damaged you may can get by with what you have if you can't get it apart. If push comes to shove and you have issues with it you can always take it to a hydraulic repair shop and they usually can do all this for you.