65 Barracuda.....what does this booster do ?

The later booster has the reservoir as part of the booster. Those early Bendix booster are built a bit different. Instead of a rubber diaphragm, it has a leather seal on a piston. Not the greatest design IMO. The Bendix booster needs that extra reservoir to operate safely. Personally, I'd remove that entire system and go with a '67-up manual brake setup unless it's going to be a factory stock trailer queen.