I have a paslode straight finish nailer, purchased in Australia around 2001. The problem is that the pin that drives the nail out of the gun does not retract back into the cylinder. I have it apart and the piston seems not to be sliding very smoothly along the cylinder. Thanks for any ideas

Is the cylinder really dirty, if so it needs a thorough cleaning. your instruction mnual detailes how to do this. also look for a jammed nail in the nose area and possibly a bent driver.

You will find that if the correct O rings are not fitted you are not making pressure, if you push the pin back in with a nail and fire and it works but is not retracting it means the rear O ring is split on not making a good seal. Purchase 2 O rings (buy the correct rings from paslode) cheaps ones will cause you more hassle than its worth, and clean the gun out with the paslode degreaser, wipe it down and lubricate with the paslode oil. put a nice amount around the o rings when you put them on. this should solve your problem.
other things it could be,
Poor gas (bad expiry date) not paslode gas
The firing pin could be bent or could be upside down. This drives the nail but pushes the rob upwards.
I think it will be the back O ring personally but check all parts when you have it apart, also a little oil on the rod support will help.
Hope this helps Jon G Wales