Thank you for the advice. The unit is used in a warehouse, so the mast channels are clear. The cap on the lift cylinder in the middle of the mast was replaced recently, so I did bleed that cylinder. I returned it to them and requested that when used they pay attention to when it was bouncing, specifically where was the carriage when it was acting up, on the center cylinder or the two outer cylinders, or after they stop lowering. I am waiting on more information.

air getting in the cylinder can cause this...
also someone mentioned the velocity fuse (or safety valve), that can cause lifting/lowering problems too, and yes there is only one way to check that, by removing it, if it's located inside the cylinder rod base then you'll have to remove the rod out of that cylinder and take it out and physically inspect it.
one other thing to consider is make sure the mast channels are free of any debris and the tracks the rollers ride on are smooth and free of any dirt or grease. And then finally there may be something wrong with the cylinder itself causing it to drag making it bounce if it is doing this while moving downward.
If this bounce is only happening when you stop lowering then that is air most likely, if the cylinder has a bleeder screw on it you'll have to bleed it out by raising the forks up, cracking that bleeder and allowing it to purge the air, when pure fluid starts coming out then tighten the screw and that should stop the bouncing. (this is only if the cylinder has this bleeder screw, if it does not then refer to the other causes mentioned).

I don't think there's a way to check the velocity fuse. That has caused bouncing on some trucks also

This unit does not have an accumulator.

Normally when mast bounces is due to a bad accumulator.