Naughty Kitty

Doing winterization on the 496s this weekend and was at the part of fogging the motors (2oz of 2-stoke oil in the fuel/water separating filter with fuel supply line removed) when I was surprised by the vacuum at the pickup hose barb. As the engine was running, I expected a constant vacuum to the tank, but what I found was for almost 30 seconds there was zero vacuum on the pickup (just using my finger over the 3/8" fitting). I was kinda freaked out, then after about 30 seconds it was sucking as expected. So this leads me to the point of this thread...are these pumps supposed to cycle during operation and if they are what sensor(s) would be in play to tell them to turn on? Reason I ask is I have been hunting for fuel delivery issue for one motor, I have pressure gauges reading the fuel rail pressure and during some garage testing I had a vacuum gauge tee'd into the pickup side (but guessing the pumps never came one during the test because Hg was at zero).