Before beginning this test procedure make sure vacuum brake booster is in good working order by following SSBC test procedure. Also check to confirm all vacuum hose connections are sealed tight and not leaking.
This procedure will require the use of a hand operated vacuum pump with a vacuum gauge. If you do not own one it can often be rented or borrowed from most “big box” parts stores. (Note: 18”HG is the minimum engine vacuum at idle in gear to effectively operate a vacuum booster