Power Steering Reservoir & Caps
 

So we plan on building a custom power steering reservoir for my project car but noticed the OEM reservoir's cap vents to atmosphere, is the common? its a detail i cant seem to find when looking at suitable caps or just buying a whole new reservoir.

The GM cap features a vernay valve inside, it's supposed to retain a small amount of pressure on the reservoir but allow air in when needed. I'm not convinced they all work as intended though.
http://www.vernay.com/ProductCategories/Duckbills.html