Our old 854 RoGator had a 1/4" line that ran from the top-most 1/8" plug on the pump housing up to the top of the tank. Never had a problem.
Our JD4830 has a similar 1/4" line but we definitely have problems with it not wanting to prime. Some have suggested in the past that when the pump runs empty, it can generate some foam that gets into that line and then the air can't push its' way out. I have heard that to help, install an 1/8" (or 1/4" depending on your pump housing) valve in one of the other positions. Then if it won't prime, open that valve to let some of the air out. I haven't installed the valve, but I have removed one of the pipe plugs and then replaced it when it starts pumping water again - takes seconds!
Not sure why the big difference between the 2 sprayers. Only difference I could come up with is that on the 854, the pump is directly under the tank and the output side of the pump was pointed straight up, past a few tee's through a big 2" valve and into the bottom of the tank. The air had nowhere to go but up into the tank! On the JD4830, if you are pointed uphill, the air can't get out of the pump except through that 1/8" fitting on the pump. Takes a long time to get air out of 3" hoses through a little 1/8" fitting! That's why pointing downhill can help - the air naturally heads uphill into the tank.
Andrew
P.S. I should note that we use the sprayer pump for filling and the water level in our truck tanks is higher than the pump, so it should flow in!