Quick question about brake booster pump

[oya]

Hello all
In the process of replacing the brake booster pump. The new part came with an O ring. Can’t figure out where it goes. Any input appreciated

 

[oya]

Hello all
In the process of replacing the brake booster pump. The new part came with an O ring. Can’t figure out where it goes. Any input appreciated

View attachment 3618998

View attachment 3618999

It fits snug into the hole in the middle but the old one didn’t seem to have one.

 

[nissanh]

That O ring might be at the neck of booster assembly (pressurized N2 tank):