I would put you mity vac on the main line from the vac pump. Plug any other outputs and then see what the vac reads. If it still leaks pull the line off the break booster, and plug it and see if it holds. If it holds then it is definetly the booster, if it still doesn't hold it's your vac pump.
Sounds like it is your booster but this will tell you for sure.

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