Rainman is right on the money. When I have some water flowing through but not operating at siphon, I have to remove the vent cap and close a valve at the bottom. The water then fills the pipe and I put the vent cap back on. Then I open the valve and let the water flow. For some reason if I keep the vent cap in place while trying to allow the pipe to fill it never gets all of the air out of the system and the siphon doesn't start consistently.

Last thing - be very very careful around an open vent cap or the intake side of a siphon. It may have tremendously powerful suction associated with it.