Skytrack Boom extension issue

ok, have a 10042, will not extend all the way out, on the intermediate boom, there are letters. Typically it will extend out past "G", right now it won't show the letter "F". The wear pads are a little out of spec as being worn. There is a hammering sound after retracting, at the time of extending, but it is quick.
Limiters appear to be working as I've disconnected and reconnected. Chain tension is about right. Could it be air in the cylinder? Any ideas would be helpful
Thanks
Ian