Your best bet is to contact Andy at Inland RV. He knows this system inside and out.
I had a '77 Excella with those brakes. In the old days, you would tap into your car's brake booster so that you had an air fitting back by the hitch. Your car's vacuum would operate the booster in the trailer just like the one in the car.
Today, most folks use an electric over hydraulic unit, which replaces the need for the air line to your car's booster. The EoverH unit senses the need to brake from the wiring from your brake controller to the 7-pin connector, and then applies the hydraulic pressure to your brakes via an electric piston rather than a vacuum operated one. I believe Kodiak is one big brand of them, and there are several more.
But unless you go full retro, that big vacuum booster on the front of the trailer is a piece of scrap and not much else.
Just the same, talk to Andy and he will get you going.
Best of luck,