compression help

You would be hard pressed to ever see 30 psi vacuum with the engine braking in a manual car downhill at high RPM! Maybe the OP meant 20? I hard to work hard at it to get more than 28 psi down a mountain in a /6 LOL !

The last post indicates that there is adequate residual vacuum in the booster just for one use that is being bled out on the first brake application, so suspect a problem in the booster's vacuum/vent circuit. If you have the throttle open with low vacuum and apply the brakes successively, you will eventually bleed out all the usable vacuum from the booster.... but with a good check valve, you ought to get 2-4 good brake applications, not 1.

Be sure that you check the brake booster on the passenger compartment side to make sure the pressure side vent filter is not clogged. Air has to flow in from that side to allow the diaphragm to work. And see if the check valve on the vacuum side is working.....do you even have a check valve?