Strange pressure drop in irrigation system.

[Eugene Wilson]

I have a new install designed by Rainbird with a 2 hp Red Lion pump. When I turn on a zone, the heads put out a strong steady stream for about a minute, then drops to a very small stream, UNTIL, I turn on the hose bib at the pump outlet pipe. There is very good pressure at the hose bib, AND, the sprinklers have a good, strong stream again.
Ideas?
Thanks

 

[Valveman]

Suction leak. Little air doesn't make much difference at high flow rates,. But using less water like a small zone will make the little bit of air a bigger percentage of the total and the pump will loses prime or almost lose prime.