Many years ago I worked for the Forest Service burning logging slash piles. We used back-pack propane torches. When the torch bottle was empty, we returned to the pickup and filled the bottle from a 100 pound cylinder in the back of the truck. Liquid transferred from the big cylinder to the little bottle using only the pressure in the big cylinder. Of course, we had to open the "80% valve" on the little bottle to relieve the pressure so it would fill properly. As soon as we got liquid from the "80% valve" we shut off the valve on the big cylinder, shut off the valve on the little bottle, disconnected the hose, connected the torch and went back to lighting fires.
This long dissertation is just to illustrate that it IS possible to transfer liquid propane without a pump. Is it "safe"? Probably not. Anytime a flammable gas is vented to the atmosphere it can be dangerous.
Good luck.
