Iv replaced three brake boosters on my 98 Silverado two wheel drives due to the dreaded hissing sound now all three iv replaced is doing same thing bleeding brakes checked hose and check valve all check out fine any help be appreciated 



If it's still hissing, you likely have either a vacuum leak from the engine to the booster, or the master cylinder is going out and needs to be replaced.  

Have you checked to make sure there is a vacuum reaching the brake booster. As Jonah mentioned maybe there is a vacuum leak.