The biggest thing is getting the air out of the pump and making sure that you do not suck air from the intake. I am a firefighter and I can tell you that a small air leak on a vacuum intake will cause a truck that flows 1500 gpm at 150 psi to loose it's prime in a draft situation. A 1.5' drop is not going to do any good if you have anything keeping the air in the pump. A few cc's of water in your coil will do that. If I am having pump primer problems, I open the top discharge on the pump. I will not bore you with a hydraulics. I train new drivers to pump.
Try disconnecting the discharge hose on your pump. It should not take more than a second or two. I used clear tubing to connect it to the MLT. If you see bubbles, you are sucking air. It may not be the connection, it may be the valve itself.