unless you can find someone with a hard copy of the parts manual for this truck i doubt you will find any info on it.
your best bet is to remove the cylinder and take it and have it repaired at a cylinder repair shop. They can find the packing or have it made for it.
This is a very old truck and the information for it is not published anywhere other than maybe the factory where they probably have it ratholed on a shelf in the corner of a dark warehouse and even they might balk at hunting it down but it might be worth a shot getting a dealer to locate it unless they are the ones hanging up on you :o/