These boosters are really good & were very popular in the years when FB Holdens still had warranty, [Early 60's] but were designed for a single circuit operation. If you want to add boost to both ends of a dual circuit brake system, then you need two boosters.

The round thing protruding downwards is a filter [with valving]
This strains the rocks out of the air that passes into, & out of, the diaphram chamber when it "breathes" during brake application. The sound of it breathing under the dash of a street car may cause some passengers to wory what kind of pet you have lurking under there, but in a racer, you will never hear it above the cheering of the crowd.

The pipe coming to the top rear of the cylinder supplies hydraulic pressure from the master cylinder. The fitting at the end of the cylinder that points upward is the bleeder nipple. The unit is best installed with the cylinder pointing slightly upward to get a good bleed. The pipe at the end of the cylinder goes to the brakes. The rubber hose supplies vacuum from the manifold, or from a vacuum storage tank. This one looks to be both well engineered, & properly installed

They work a treat but were superceded by the type that mounts the master cylinder as this design needs only one booster to work a dual circuit brake system.

PS
If you find one in a junk car, don't throw it out, take it to a swap meet. They are still popular & some places specialise in reconditioning them.

What year is your ute NIS15L, its rust free

I've never seen this before.
Can you please briefly explain how your brakes work. And what happens if your engine shuts off?