I either have a failed vacuum pump or failed brake vacuum booster.
How can I tell which?

1 When I disconnect and plug the main vacuum hose at the brake vacuum booster, I get strong vacuum pressure (15 psi) and all functions work fine!
2. When I reconnect the main vacuum hose to the brake booster, I barely get any vacuum functions such as shutoff or door locks and vacuum pressure is about 4 to 5 psi max.

3. I either need a new vacuum pump or brake booster. How do I know which?