Vacuum fluctuates when brakes applied

Here's your booster test;
with the engine off,pump the pedal several times to equalize the chambers. Then depress and hold the pedal with modest force, and start the engine. Immediately the pedal should go down about an inch.
If it does not do this, then either the pushrod adjustment is too short, or booster is not being evacuated. If you hear hissing under the dash while applying the brakes, then the control valve is busted.
If your engine idles at over 15" vacuum, then that is enough to operate any Mopar booster.

IMO your diaphragm is ruptured in a place that gets aggravated with pedal application, or the control valve is busted. Hearing the hiss would be the proof. The hissing is easier to hear from under the hood,right next to the firewall