The pump bypass valve in any Weber operates in same manner.
The ball "should" be unseated at rest, since it has to be raised from pump circuit pressure to seat.
http://www.poustusa.com/weber_carb_pages_pix/weberd.gif
That's why I run steel not nylon bypass valves.
Plastic balls seat faster and give a more responsive pump circuit, but they also are more prone to the above complaint.