Fluid should not be up there above the piston, air only. You'll probably need to remove the bed cylinder and get it rebuilt if possible. If all it needs is a new piston packing, not much of a problem- - - -if the cylinder is scored or rust-pitted, things get pretty $pendy in a hurry.

Edit: What weight of fluid are you using? A lot of the older dump trucks I've worked on used a very heavy hydraulic fluid, something about the thickness of 40 weight motor oil. If you're using something thin like EP-32 tractor hydraulic fluid, a seep leak will turn into a gusher.
Jerry



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