The hex shaped brass thingy with a hole in the top is the bleed valve. It could also be called a purge valve.

It's purpose is to bleed off any air that will get into the pump after extended non-operation. This is to prevent a vapor lock condition from developing in the oil pump.

When installing a new oil pump, the proper procedure is to prime the pump on the workbench. This priming does two things:

...it flushes out any debris left over from manufacturing [hopefully none at all];

...it coats the gears and housing with oil to improve suction during initial start up.

And there is one other thing that priming the oil pump does, but which is usually overlooked:

...verification that the purge valve functions correctly.

The purge valve should be open initially; as the air bleeds out, some oil will also bleed out. Then, as pressure builds, the purge valve should close, stopping the oil leaking out.

If the purge valve is stuck closed, oil pressure may never build [the extreme condition]; or take a long time after start up to build up: vapor lock.

If the purge valve is stuck open, oil pressure will build; but be slightly lower than it should be.