If it were the check valve, it'd have no impact on your issue. The check valve is really there to ensure you can brake at least once in an emergency if you lose engine power. If your booster diaphragm is going bad, it will allow the engine to suck an infinite amount of air - check valve working properly or completely missing.

The only way to really know is to hook up a vacuum gauge to your manifold and have someone apply the brakes. It should show a small pressure increase initially as the booster demands a stronger vacuum, and then level off. If it doesn't level off, or the pressure keeps increasing in your manifold, the booster's shot.

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Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k