Pumps always go on the lowest point, like Squeeky's example pointed out, it's airlock galore otherwise.
Name one supplier that doesn't have them at the lowest point in their systems. Says enough.
Another thing that helps is to keep the vacuum side to tank outlet as short as possible. And get a ball valve as a tank outlet. If you are running low on water, turn the stop in between jobs when driving, to prevent air from entering. This is number one cause of airlocks. Make sure to open it again when you stop though!
Pumps suck about 3ft, but if that's filled with air, your going to have trouble.
If air does enter, unplug hosereel, put pump at highest setting, and let the air come out. Then connect hosereel and run like normal.