Looks like a Commercial Shearing or Intertek dump pump with an air shift PTO, and controls. In your photos I don't see a strainer and you may need to remove the top cover the reservoir for access. Let me see the top of the reservoir. If there is a plate large enough to remove and reach to the bottom, there should be a strainer. If just a fill cap, then there would be no strainer included.
You do not need to raise the bed to bleed the cylinder. It would be easier and less messy to have someone watch the open bleeder port as you commanded the bed to raise, but you can do this from the truck also as when it starts losing oil from this port and dripping down the cylinder, you'll know you've purged the air. Tighten the port, raise the bed full stroke three or four times, break the bleeder port loose again, and see if there is any air. If not, button it up. Air will always rise to the top of the cylinder and a leaking gland seal is always a culprit. There is a vacuum created as the cylinder retracts and older seals do tend to pull a bit of air into them with age. They don't always leak fluid on the extension cycle either. Your cylinder looks as if it's leaking a bit from the photos so could be time for a repacking. I don't see a filter assembly in your setup as it would need another line only used on the return side. You only have a single line system meaning a suction and return line from the reservoir, and a single high pressure line to/from the cylinder. Both lines are used in both capacity's. Most systems incorporating a filter use a separate line for return to tank and this is where a filter would be incorporated. Doesn't appear you have one.
