Just wondering if anyone has a simple test to check brake booster function.

I have a '92 Civic VX - brakes fine and normal as of now but hearing a slight hissing sound
when I apply the brakes at low speeds. The Master Cylinder is a NAPA replacement
from years ago (10+), and my instinct is that this is the problem (and NOT the booster)
and needs to be replaced soon.

Symptoms : When I turn off the engine the brake pedal hardens up right away.
When I crank up and there is vacuum the pedal releases and is soft.

THANKS !!!