I have a 1995 Blazer and a couple months ago, I noticed that the brakes were kind of spongy, more so than normal so I changed my calipers, pads, rear shoes, wheel cylinders, and brake booster. Now, none of that made much of a difference so I changed the Master cylinder today and bled all my brakes, but now I am hearing a sharp hiss of air when the brakes are pressed. It happened more so when me and my friend were bleeding them because I was actually under the hood checking the MC to make sure there was no leaking, but I don't hear it so much inside. Now, the brakes still seem kind of spongy and the pedal just seems like it is going to far to the firewall. I checked my lines and I am not seeing a leak anywhere. I am not a mechanic and every time I take it so a shop to get looked at, I get screwed probably cause I am woman, despite the fact that I know a little so I don't trust anyone with any of my vehicles so I am on my own for this. Can anyone offer some advice? I hissing sound is definitely coming from the driver front area, but it doesn't really sound like it is directly from the MC. I don't know and snow is right around the corner so I really want to get it fixed.
Thanks

 

Sounds like it could be the brake booster, there is a main vacuum line that runs to it, check and make sure that is on and that the fitting on the brake booster is not broke. To check and make sure its not the booster itself you can start and let the vehicle run a bit then shut it off, depress the brake pedal, it should not be hard right away you should have enough vacuum in the booster for a couple pedal applications before the pedal goes hard

 

Another vote for the brake booster.

 

Hm. Changed the brake booster, though...and the usual result of a brake booster failure is a hard pedal. I don't know if there's anything else to change, though, and the loud hiss does seem to point that way.

When does the hiss happen? When you're pushing on the pedal, or all the time? And does the engine run rougher while it's hissing? YOu might also try pinching the vacuum line to the brake booster and see what that does for you.

 

jmaynard x2. A failed booster almost always results in a hard pedal, not soft. The first question is did you bench bleed the master cylinder before you installed it? If not, remove it, bench bleed it, and start over. Second question is did you get the right booster? There are more than one type. I did a job a while back where the guy bought a junkyard booster, and while it was the correct type, (theres an SD model and an SU model), it didnt come with a piston. I took the piston from my truck which had a different model booster and we eventually found out that the two had different sized pistons. Mine was longer (lol) and it not only made a hiss, but was always slightly applying the brakes, with made his braked seize up after a short drive. So check your old booster and make sure youve got the right one. There should be a white sticker on the front somewhere.

 

If the booster is leaking vacuum (the sucking air sound), it could still be doing what it is intended to do. Air in the brake system may also be an issue, but the vacuum leak points to the booster (whether incorrect application or malfunctioning).

 

Also, changing your master cylinder will definitely allow air into your ABS unit. I cant remember if your ABS unit has bleeders on it or not. If it does, youll need to bleed it. If not, you have a couple choices. One, you can take it to a shop that has the ability to bleed your ABS unit with a scan tool. Two, you can go old school. An old trick is to take the car out on a dirt/gravel road and intentionally throw it into a skid while slamming the brakes. This will cycle the ABS and any air in it will be ejected from the unit into your brake lines. Then you just need to bleed the brakes again. It may take several skid/bleed sessions before all the air is purged. Just be very cautious when attempting this, as your brake system is compromised, and could be dangerous on the street.