OK subject is an 83 k5 with crate sbc 350 (35,000miles), 700r4 with np208. I replaced the booster about 3 months ago. Then noticed a hissing when applying the brakes, as well as shaking while at a stop. Replaced all vacuum hoses. Hose to booster Pcv hose Heater hoses as well as booster check valve, pcv. Still I get hissing when stepping on the brakes. I'm getting about 15" of vacuum through edelborock 1406 manifold port side at 750 rpms. Timing is at 17 initial 34 total. I live at 5500 feet but work at 6500 feet. Daily driver. I'm on the 3rd booster all from Napa When I spray carb cleaner around the intake, or any possible vac leak locations I get no change in idle speed. The base of the carb I get an increase in idle. Now I know I need to replace the gasket there. What else can cause my booster to leak air when applying brakes. The sound can be heard underneath the dash when.. I'm almost at a lose here Any help would be appreciated. I'm a 911 operator and retired Marine, my next day off will be this mon (memorial day) an Tuesday after that it will be the following weekend. Where should I start other than the carb gasket Memo.