Yes there is air against it always. That tailgate valve Lantraxco posted would work for just up and down. If you ever needed to leave your bed up and get out to see if it was clean, or put a block in your tailgate or anything, and you didn't take the PTO out of gear, you have to buy another pump. What Overworked was talking about is that when you switch from the up position to the down position, if the valve couldn't let the air out of the "up" side, nothing would happen. That tailgate valve does have exhaust ports.
I would still strongly advise against using it, I would spend the extra, even if it's just a farm truck, because if you just raise it up and go to pull forward a little, you've got to stop and take the PTO out, or the engine's going to be straining to lift the bed farther than it can go. That's not good for your pump, cylinder, hoses or anything.
Even if you have a neutral position, leaving your PTO in gear for a while will overheat your pump, unless you have it setup with the third hose.
I once had a very good driver burn up a 1 day old pump because an idiot landowner came up and started having a screaming fit. The driver left the PTO in gear with the valve in neutral while he calmed the man down. The pump got so hot it melted nylon air lines near it. After I changed that pump twice in 2 days I put third lines on all my trucks.
In short, the pump running in neutral will ruin the pump after a little while, the pump running in raise, when the bed can't go higher, will ruin the pump almost immediately.