Fuel in vacuum advance unit

[MarkyB]

There will be lower pressure but no flow of air as it is a dead end.
If the diaphragm was split then there would be air flow.

The early metal pipes had a chamber in the middle probably to stop petrol getting to the diaphragm.
This was discontinued when they started using the plastic push on pipes so maybe it isn't worth worrying about.

My guess is that the relative coldness of the vacuum advance causes petrol to condense in there.