Still need help on vacuum problem. 300 SD 1985
Narrowed down to brake booster or vacuum pump or check valve by making a test hose from vacuum pump to brake booster with no joints or branches. Vacuum still was slow to build up and vacuum power brake went away after 4th brake application in quick succession.
When vacuum was measured, it did reach 15 to 20 at the check valve in the black main vacuum line.

Replaced O ring at booster, no change.
Removed a brass tube with screw threads from vacuum pump. I thought it was a check valve but it seems to be a hollow brass tube with screw threads. Is this normal?
See pictures 1, 2, 3 below.

Vacuum pump where the brass tube was removed has a silver color object in the hole so I cannot see into the vacuum pump. Is this normal? See pictures 3 and 4 below. See that vacuum pump is the black greasy object with silver color screw threads and a silver color metal object inside the threads near round power steering cover.

Is the brass tube a check valve? Should the hole in the vacuum pump look like in the pictures?

Thanks.

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