[SEL] Q on engine at Portland

[Rick I.]

In this Portland picture:
<http://www.oldengine.org/members/holland/images/Portland2005/P8250021.html>
(very nice set of pix, by the way!!), there is a small brass petcock valve
on top of the engine cylinder.  What's this used for?  There isn't a cup, so
I don't think it's for priming- or is it?

Thanks!
Rick
(Rick's Taylor Vacuum Engine: http://www.oldengine.org/members/inzero/ )