Austin 16 engine block drain tap

Hello All

Austin 16 BS1 1948 2,2

I am in the process of flushing through the waterways in my engine block, the engine is still in the car.

when I tried to drain the block, only a drip of water came through the brass tap, so obviously there is a blockage, and I will try to sort that out.

What is confusing me is the brass banjo connection that is part of the drain valve. It has a rubber pie fitted to it. I can’t see a reference to it in my manual, and I cannot work out what it is for.

I am sure this must be common with other engines, can anyone tell me what this item is designed to do?

Thank you. (any advice about how best to flush deposit build-up from my engine block will be well received.)