3 things....
- what serial number is the truck?
- what is the mast number?
- which cylinder are you speaking of?

there are usually 3 cylinders to deal with on a mast
in the case of the center cylinder you just remove the crosshead and there is a cap that unscrews out of the top part of the shell. They usually require a spanner wrench to remove but you can use a hammer and punch to break it loose and then use channel locks or a pipe wrench to get the cap unscrewed.

and as for the outer or secondary cylinders there may be a bolt that screws down through the mast crossbar and holds it to the mast crossbar but it also has a screw on cap generally.
these caps on either cylinder contain seals that come in a seal kit usually.
IF it is the outer cylinders they are alot more tricky to remove as you have to partially raise up the mast (about 2ft or so) and block and chain it, then remove the bolt holding the piston at the top and then lower or retract the piston to get clearance to remove the cylinder from the mast.

now as for the piston(s)? they may or may not have seals on them, you will have to remove to see, in any case it should be removed anyway to check the safety valve in the end of the piston.