My 2 cents
If your pump doesn't reach 20 inhg in 35 seconds from start up , you have a bad pump , but your hose running from your pump to the fitting on the fire wall should be connected on my to a gauge (detached from system) if it takes 30-60 seconds you have a weak pump .
If you have vacuum (15+) and you watch the gauge as you step on the brake and it drops immediately to ~5 you have a bad booster.
Next you need to detach each line individual at the 5 way by taking the hose off and plugging It with the vacuum gauge , then start the truck and see if your vacuum numbers change , higher or lower then turn the truck off and let it sit and see if they system leaks out . This will help identify which line is leaking.
And you also need to check your one way valve , this is the item that sits on the booster , I would just change it but to check if it's working , disconnect all lines and plug them except the one that goes to the booster/check valve , (and the vac pump line that goes to the 5 way obviously) have it our gauge plumped into one of the 5 way and run the truck till you get to fill vac then turn the truck off and see if it drops , time/how much vac your loosing will determine if your check valve is any good.
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