Thanks very much, you were spot on! The bushing took a little bit of coaxing but did come out relatively easily. Glad I did so, because with the amount of gunk and corrosion I found inside the cylinder, the booster surely would not have worked properly once installed.
So I think I understand how this thing actually works now, someone please correct me if I am wrong.
Under normal operation (no braking), the vacuum is applied to the front side of the booster "can" via the banjo fitting/air passage, and also to the back side of the can via the air valve (pentagon attached to the hydraulic cylinder) . With both sides of the can at equal vacuum, there is no assist to the brake. Once the brake pedal is pressed, the hydraulic pressure increases, and moves a small piston that actuates the air valve . This new position of the air valve no longer allows vacuum to pass from the banjo fitting to the back side of the can, and simultaneously allows ambient air pressure to the back side of the can (via the small "filter" on the air valve). Under this condition, the two sides of the big can are no longer at equal vacuum, causing the diaphragm to move inward and push a small rod/piston into the hydraulic cylinder, causing more hydraulic pressure, and effectively giving assist to the braking.
The arrow in the attached picture points to (as I've assumed) the small piston that actuates the air valve. Is there any way to clean/service this part? On the brake booster currently in my gtv, I am pretty sure that this is the part that is sticking (in the "up" position) resulting in vacuum assist sometimes even when the brake pedal is not pressed. Since the inside of this hydraulic cylinder was quite dirty, I want to ensure the same thing doesn't occur.
ps it looks like there is still a check valve in the cylinder, does this come out somehow also?
69 1750gtv / 73 2000spider / 91 Spider (daily driver)