Do you wait the 20 seconds before pressing on the brake, or is the test done immediately upon startup?

I'd be fairly sure that the booster requires 15-20 seconds at idle to completely evacuate. In fact, I hear some of the vacuum doors operate after about 15 seconds or so on the SD.

But, any time outside of this window might indicate a weak vacuum pump.

Pretty simple to check. Put a vacuum gauge on the main line and start it up.............