"T" a vacuum gauge in the large hose between the intake and the booster. (You'll have to get a replacement hose later.) Sounds like the booster isn't getting or holding a vacuum. Could be plugged or collapsed hose, bad check valve at the booster (that plastic thing that the hose connects to) or bad booster.
At the big bad oil company we had a test oil with high volatility, lots of vapors coming back through the PCV system. The deposits in the intake manifold were so bad that the brake booster ****** in the manifold plugged - no power brakes!
