master cylinder hiss

I recently replaced my calipers, hoses and pads... I couldn't get my front brakes to bleed properly. I changed my master cylinder by recommendation of my buddy who owns the auto parts store in my Hometown. I bought a new Delco master cylinder, now when I press the brakes, there is a loud hissing noise. The truck stops well, no pulling, or locking up, or brake light on my dash. Any ideas? Thank you...

vacuum leak at the brake booster.

I forgot to mention that it will hiss with the truck shut down. Does the system hold vacuum without the engine running? Also, when the pedal is pressed hard the hiss becomes more of a squeal.

probly a vacum leak, my moms car did the same.. just kept getting worse and worse

well i would like to know what year vehicle yes they do but if it hisses ofter you shut the truck off its probally brake fluid making that noinse yes the system hold vacume after you shut it off but only for a couple hits try pumping the pedal after to see if the noise goes away after a few pumps the pedal will get hard as soon as you loose you vacuum

It is a 1997 k1500 350ci. It will continue to make the noise as many times as I would like to press the pedal after the truck is shut off. Is it normal for the fluid to hiss? I have never heard it on any other vehicle including this one before I changed the master cylinder. Thank you for your help.

ok so when you did all this you properly bleed all the air from the line and now your experiencing a squeel try taking the hose off the motor not the booster pack adn try without oddly twisting the hose out of its place in the booster anotherwords follow the hose to the back of the intake disconect it then pull it around without pulling the hose out of the booster and try sucking on it and creat a certain amount of presshure and see if you can get any air out of your booster c if it will hold pressure if not then its bad if so then your probally looking another master cylendar can i ask why you changed your barkes int he first place did they fail or just normal wear and tear

I had the same problem with my 97 also. On mine the problem got worse and worse over time. It ended up being a worn diaphram on the inside of the booster. Needless to say I replaced it and all is well.