AJ's guide to Transfer Port Synchronization

The booster chamber is supposed to be sealed, except where the big vacuum line enters it. Here the line is attached to a fitting with a check valve in it.
So,you could remove the valve and then Tee into that line, and install a vacuum gauge.Next,start her up, rev her up a bit to say 15 inches of vacuum, and then clamp the line completely shut. Let the engine return to idle and shut her off. Ideally, the vacuum in the reservoir should remain there forever. Practically, I would guess, a 7 minute test with little to no pressure loss would be fine.
Now, go step on the brake with moderate pressure, and hold it there. The vacuum should fall to a slightly lower level, but then remain steady;as steady as your foot remains steady.

Alternatively'
put a tach on the the engine.If you have a lumpy cam, rev it up until the idle speed is reasonably steady, say 15 inches of vacuum.Then, clamp the booster line completely shut.The engine rpm should not change at all. Then release the clamp, and again the rpm should not vary at all.Well, you might get a tiny dip and then back to where it was.
Next remove the clamp, wait for the vacuum to stabilize and step on the brake, and hold. The engine might drop up to 30 rpm or so and then jump right back up.Release the pedal, same result.
And finally
Charge the booster;( start the engine,bring the vacuum up to 15 inches), clamp the line completely shut, and shut off the engine. Now begin pumping the brake pedal.Notice on the first pump, you can feel the assist. By the third pump, the pedal is getting harder, and not traveling as far. By about the fifth pump the assist is probably gone, you have exhausted the vacuum chamber.
And one more. Put everything back to normal. With the booster exhausted, depress the pedal with moderate pressure and start up the engine. As the vacuum comes up, you should feel the pedal fall away under your foot; that is normal.