leak test brake booster

[Murphy IRE]

I'm trying to locate vacuum leaks on my 71 K20 400 sbc. I beleive my brake booster may be leaking.

I removed the hose from the intake manifold and hooked up a hand pump . After a bunch of pumping I was only able to pull 15 inches.

I did a search of the threads and did not find an answer to my questions.

They are:

(1) how much vac should I be able to pull on the booster? Does 15 seem low?
(2) once I pull the vac, the booster holds it fine.

Am I missing a step to the leak test? Or does my current test indicate that it is working properly?

Thanks for the help.