I have a question about a Bender Milk Releaser/Receiver.

What is supposed to be on the bottom glass port?
The metal rod sticks down into it but there is nothing else there. What is to keep the sap in the jar until it's full?
Am I supposed to put a check valve there myself or is something missing that I should have gotten with it?

I will post pics later today of the receiver, just don't have the time right now.

Thanks for any info