Well, I hate to have you make a mess but it makes a huge difference in what type of cylinder you have. A standard upright (2 stage) cylinder is very simple to repack, a full free lift cylinder is a completely different animal which is pretty complex. The difference is how far the forks rise before the inner channel begins to rise. A standard upright will only allow the forks to raise approx 12" before channels raise, a full free simple will allow the forks to raise the entire height of the inner channel before the channel begins to move.

To be able to answer your question I need to know which upright you have

This is a pretty basic fork lift - nothing fancy, being a model S40C. So there is just one channel that raises up when raising the forks. I'm not sure about your question #2, since if I turn it on to find out, I will get lots of leaking fluid out the top of the mast where the old seal is located. But, I think the forks have to rise pretty high before the channel starts raising.

It depends on the style of upright that's installed. When you raise the forks fully- how many "channels" raise- 1 or more?

How far do the forks raise before any "channels" begin to raise?

All depends on the mast installed on your truck. Some are reasonably easy, others are a little harder.