2000 Chevy Astro A/C

2000 Chevy Astro 6 cyl Two Wheel Drive Automatic

A/C will not come out vents unless I first put controls on "VENT", let the air start blowing through for a minute or two to get a hard air flow, then gradually turn on A/C, beginning with "least" setting, working my way to MAX. In the morning when I first start my van, there does not seem to be a problem. It reminds me of the principle of "siphoning": if the liquid does not allow a vacuum, flow continues; once air gets in the tubular space, everything has to be re-primed.

Thanks!