Pull the booster hose directly at the booster - do you hear air sucking in almost instantly as the hose comes loose?
If not ... hose leak, check valve suspect, or internal booster leak to atmosphere. Use a vacuum hand pump and vacuum gauge to confirm hose and valve hold vacuum.
Basic booster test:
Foot on brake pedal. Start engine. Pedal should sink a little bit. This is good, no sinking, not so good.
Now release pedal.
Stop engine.
Pump pedal a few times until the booster effect is bled off. Should take a few pumps.
If not ... more possible booster issues.