As a general question, how much internal leak down is acceptable without running the risk of causing a major failure as you described? For reference my Cat 315's thumb will weep down, not 12" in ten minutes, but it will drop to the ground overnight from the folded position. I have a couple other functions on different machines that will weep down, or sag if they are holding a load.
Also, what exactly do you mean by "cratering" the cylinder? What would happen internally that would cause this, and what exactly would be cratered, the cylinder walls?