you better watch out cause mine did the samething was making a hiss noise brakes felt a little funny then i got home went to leave a little later and my brake booster was done my truck would stall, and i picked up my brake booster for 100 all you have to do is take the old one out bring it to autozone or something & buy your new one. it kind of a pain in the *** to get 3 of the 4 nuts off but saves money haha!

 

YES, Your booster WILL GO BAD! I had this same problem, eventually stalling the vehicle when the brake is applied. I replaced with a new one, and there was NO HISS. The hiss is a sign it is going bad. If it starts to effect engine speed, you have days at best.

 

what about in high rpms my truck around 4000 rpms starts to slow and then about 4700 its back to normal floored rev

 

Glad you found out it is okay. I was going to suggest you look at the large vacuum line going from the intake manifold to the brake booster. If that large vacuum hose is bad, it will cause a 'hiss'. It is subjected to a lot of twisting and flexing because the rubber hose goes from the shaking engine to the rigidly mounted Booster on the fire wall. It seems they always split on the other side of the hose that you cannot see. They are relatively inexpensive and easy to replace.