Power brakes are a creature comfort to reduce pedal pressure applied by the driver.
In other words, you can disconnect the vacuum booster and plug the vacuum line to the inlet manifold and the brakes should still work but with increased pedal pressure.
If your pedal sinks to the floor, that indicates that the hydraulic system is at fault.
If the brake booster doesn't have a one way valve in the vacuum line, it may be worth while fitting one.
The valve allows the tank to store vacuum and prevents the vacuum from bleeding away.
The vacuum advance line to the distributor can be temporarily disconnected and plugged to aid in the fault finding process.
You may have a faulty vacuum actuator on the distributor which could be allowing the vacuum to escape to the atmosphere.
Not quite scientifically correct but you get my drift
Vacuum advance on a distributor is a fuel economy device and does ZIP for acceleration and full throttle performance.