Is there a foot valve incorporated into the strainer on the end of the suction line? IF not and you are trying to lift water, you probably need one. You need to fill the entire suction line with water and the foot valve(a large bore flapper check valve) allows this by keeping the fill water poured in at the pump from running back into the pond. Just like a siphon won't work with air in the line, a pump will also quit with too much air in the line. Any air in the line will stretch instead of transfering the suction force to the water below it. This air will stretch till it reaches the pump impeller then you will loose any suction you did have as the pump impeller spins in air. Like any hydraulic system, it is far easier to draw air in thru a leak than to suck fluid thru a line so the suction line must be airtight.
I used to set up emergency pumps onboard ship and we would accomplish the suction line prime in one of 2 ways. With a line attached to the top of the suction hose(where it attaches to the pump inlet), we would throw the whole suction line over the side and allow it to submerge and fill, then we would pull up the suction hose(full of water, held there by the foot valve) and connect it to the pump. The pumps we used had a small hand suction pump to finish lifting water up the suction line to fill the pump housing and finish the prime. Any leak where the hose met the pump and you couldn't finish the prime with the hand pump. These pumps also had a large funnel fitting on the pump body with a sealable hatch. You could drop a bucket on a line over the side and fill the pump and line down to the foot valve using buckets poured down thru this funnel. Any significant air leak in the line or trapped airpocket that reached the pump, and the pump would loose prime...
