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?

You need a new brake booster.
The test you ran tells me the booster is leaking.

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Thanks Mr Hunter

I would agree it's the booster.
To make absolutely sure you can temporarily plug the vacuum lines that tee off from the main. If the brakes are still hard then....

Danny

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15 psi of vacuum pressure? Interesting measure.

I expect over 20 inHg at the pump so if you're getting 15 inHg I'd keep an eye on it.

Sixto
95 S420
87 300SDL

My shlomo has hardbrakepedalitis as well, it has as long as ive owned the car.

my question about this problem is thus:

can/will a leaky brake booster cause vacuum door lock/shifting/shut-down problems as well?

i couldnt ecavtly track down a vacuum leak in the lock system and i am wondering id maybe i was looking in the wrong spot?!

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I think it was Shakespeare who said "A vacuum leak anywhere is a vacuum leak somewhere." Or maybe it was Yogi Bera.
Yes, a vacuum leak in any vacuum component will "steal" the vacuum needed by other vacuum-operated devices. And the brake booster is a major user. Witness the size of the brake booster line.