Brake booster check valve

Have a 1964 B Body that the brake booster Midland Ross check valve might be bad.
How do you test and were are the replacements found.

Thanks

Easy check run the engine so it fills with vacuum, shut down pull the hose off if bad you will hear the vacuum sucking through the hose. If good pull the valve from the booster and you will hear it sucking through, nothing if bad