What's this ball for?

[labrado]

Today I replaced the water pump and its gasket of my 2.4 puma. After disconnecting the vacuum pipe from the vacuum pump, I find a plastic/rubber ball on the ground. I suppose it resides in the brass fitting of the vacuum pipe, can anyone confirm this? And can anyone tell me what's the purpose of the ball?
Thanks
Weihttps://uploads.tapatalk-cdn.com/20190112/d7def204fe0b129ef8f4f78b7793e96f.jpg

[Blknight.aus]

check valve