power brake booster

My experience with that booster and a 292/509 is not good. With a [email protected] it was not bad. With a [email protected] I threw it in the corner for a bigger one off a 73 Dart.
The check valve, if good, should easily hold enough vacuum for at least one stop from 60mph with the engine shut off.Sometimes they hold vacuum for at least 10 minutes after the engine is shut off.
I fear yours has a bad diaphragm, or a bad control valve, or the pushrod adjustment is preventing the control valve from seating.
If your engine runs rough with the brake applied,or the idle jumps around when you pump the pedal,it may be sucking air through a perforated diaphragm.If no change, then I would loosen the booster and space it towards the rad, about a quarter of an inch and let it sit there, at it's normal attitude.Hang it from the underside of the hood if you need to, or block it up. Then charge the booster by running the engine for a half a minute or more, at fast idle. Shut it off, and wait for ten minutes; then check for vacuum, by manipulating the check-valve. If none found I would assume the control valve is bad.
I am much happier with the large diameter single diaphragm.