While energized the two nipples are a pass through, de-energized, the open side vacuum circuit is bleed off through a hole in the end of the solenoid, a foam filter block was glued over it originally to suppress the noise. The A/G systems vacuum reservoir has a check valve on the manifold side to retain enough vacuum for several up/down cycles after engine off, on a good system with no leaks. BTW, this type of vacuum solenoid valve is pretty common in vehicles of all makes over the years for other functions, if originality/appearance is not a huge concern they can be sourced at salvage yards pretty cheap.