dump bed ???

This might be simple stupid here but have you checked the shutoff valve on the fluid return line? Have seen it more than once where they would be partly closed and cause this problem. All wet kits should have a shut off of some sort right at the tank. I put heavy zip ties around them to keep this from happening. Although the ball valve type ( handle) are easier to close accidentaly the gate type ( screw handle ) can also close partly because of vibrations.

 

When there's air in the cylinder an empty bed will bounce on the way down.

 

^^ also a dead give away with air is the loud bang n pop when coming down

 

I agree but not always true i have a DM mack when it had the old R&S body. The piston would leak down if the truck sat for a few weaks not a lot, but enough that the body would kreep down all the way. Drove us nuts when it first started doing it after bleeding it would work fine untill it sat for a while never made good sence but a little air did cause trouble!! we installed an auto bleeding valve it lasted about one season before the piston failed. The seals were shot. It was an old stile piston many of the new pistons ar what some people call upside down pistons they have a lot less trouble with this.