On a GT injection setup I checked for vacuum leaks by removing the intake tubing from the throttlebody, running the engine, then placing my hand over the TB until the engine stalled. I kept my hand there for 5 mins and still there was a good vacuum so deemed it to be ok. However, I was pretty stupid - don't use your hand!! Use a piece or rubber or foam which will hold the seal and not disintegrate under force. It left a nice mark on my hand for an hour or so :/