In the 'upright' AC type pumps the glass bowl is the pulsator dome. The air pocket at the top is a vital part of the diaphragm and valve operation.. it provides an amplification effect (increases the vacuum) so improve performance.

I have a free copy of the AC fuel pump rebuild manual at:

http://www.tm9-801.com/tm9-1828a/index.php

Read the first chapter or two.. explains the workings in all its gory details.