Good point. And this application has those type problems.

I have seen bouncing with heavy loads destroy the packings as you describe. I would advise an accumulater in the hoist cyl circuit to alleviate this type of problem in the future.

Thanks for the response SM. Problem was repaired about a week ago. I determined it was not a mast mechanical binding problem, and felt the oil was getting on top of one/ both the secondary lift cylinder pistons and the center check valve was stuck or blocked, causing the presumed oil on top of the pistons to "fluid lock" the cylinders. I ordered the kits and hauled to the shop, removed and disassembled cylinders. What we found were both piston seals blown, cracked up. However, the check valves were fine, and no debris was found in the hollow section of the tubes. This is a common failure on lots of OEM cylinders, and Cascade as well. But never with the "loading" symptoms we had on this particular machine. What I did notice was the hollow cavity on these tubes was only about 12" deep. Not the entire tube as is common. My theory is that a bigger hollow cavity would allow more oil/ air to push internal when the cylinder begins to leak- allowing a cushion of air to compress. The smaller cavity- not much ability to compress. Would love to hear some feedback. Thanks all.

sounds like you have a small dent in your first stage cylinder (seals chewed up) ,run your hand on front of cylinder barrel, it does not take much to cause misstage

I am thinking also that it is mast binding,especially if heavy loads are involved.

Kevin, there is a lot of play in the mast. This machine only has 300o hours on it, but it carries heavy, very wide loads over uneven ground. I suspect it could be a mechanical bind on the mast. I have sprayed mast lube on the slide areas.

How are the mast and carriage rollers,any broken and hanging up in the channels? Rails worn? rollers may be bad or needing shimmed if the rails are worn to much.
What leads me to think this is your comment, "significant clearance between the inner and intermediate channels (FFT). "

rogeriwt:
I have repaired some toyotas with that problem,changing the seals of the lift cylinder and the check valve in the bottom of the ram,
maybe that helps.

Have you checked the fluid and filters?