power brake booster questions

[40cpe]

I was checking spark plugs today and the #4 and #8 plugs are showing a lean condition, very white. I've recently replaced the intake gaskets when I resealed the valve chamber cover, so I'm confident that it is OK. I do have a 7" power booster for the brakes connected to the top of the intake manifold behind the carburetor. I'm wondering about the possibility of the booster leaking. The brakes work good, with the exception of there is no reserve if the engine stops. There is no external vacuum reserve.

How can I check the booster? Should it hold a vacuum if I pump it down with a MityVac? The engine is a 272" Y block and holds about 17-18 vacuum. The possibility of a vacuum leak just hit me tonight and I would like to troubleshoot it tomorrow. I think I'll start by plugging the booster fitting and see if the engine vac goes up, and trying to pull a vacuum on the booster. What are your thoughts?