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When the brake pedal is released, there is a hissing noise that can be heard in the cabin and from the engine compartment. As soon as i press the brake again it stops. It used to be very loud, but now it is just a faint hissing. Any ideas? The sound is coming from the driver's side, right near the firewall. Thanks for any input.
 

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Sounds like it could be your brake pad screecher.

Pull the front wheel off and see how much pad you have left.

If it's only a couple mm chances are the little metal arm on the brake pad is rubbing the rotor, telling you to replace the pads before you get to metal on metal.
 

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Maybe a bad wheel bearing?

Jack the front end up and with the car in neutral rotate the wheel, so if you can still hear the noise.
 

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Maybe a bad wheel bearing?

Jack the front end up and with the car in neutral rotate the wheel, so if you can still hear the noise.
Still makes the noise when the car is not moving so that probably can't be it. I'll check the vacuum lines under the engine cover. It does seem like it is coming from the throttle body/ brake booster area.
 

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I tracked down the sound some more and it is coming directly from the brake booster black metal case. Do i need a replacement or just a fix? There's no decrease in braking performance that i have noticed
 

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It's a replacement. You can DIY it's just a pain in the ***.

Here is the link I had bookmarked on replacement: http://www.saabcentral.com/forums/showthread.php?p=1250205

Get yourself a set of line wrenches for when you have to remove the brake lines from the ESP module. I had line wrenches and still thought I was going to strip out the nuts that hold the lines down.

Line wrenches:
 

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It's a replacement. You can DIY it's just a pain in the ***.

Here is the link I had bookmarked on replacement: http://www.saabcentral.com/forums/showthread.php?p=1250205

Get yourself a set of line wrenches for when you have to remove the brake lines from the ESP module. I had line wrenches and still thought I was going to strip out the nuts that hold the lines down.

Line wrenches:
Ok. I will check all hoses going to the brake booster. Then i'll see if I can replace it. It doesn't seem to really affect anything else. Does it need to be replaced right now or could I wait? What could happen if i don't replace it?
 

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there is a DIY where you don't have to undo any brake lines...you have to bend them slightly out of the way though.

if you have a mechanical pump you need to check the lines to make both it and the check valves aren't bad, otherwise you'll just be doing the booster again soon.
 

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It's a replacement. You can DIY it's just a pain in the ***.

Here is the link I had bookmarked on replacement: http://www.saabcentral.com/forums/showthread.php?p=1250205

Get yourself a set of line wrenches for when you have to remove the brake lines from the ESP module. I had line wrenches and still thought I was going to strip out the nuts that hold the lines down.

Line wrenches:
What will happen if i don't replace the booster? Nothing seems to be going wrong other than the noise. Could it completely fail and stop the brakes from working?
 

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What will happen if i don't replace the booster? Nothing seems to be going wrong other than the noise. Could it completely fail and stop the brakes from working?
I realize this thread is old, but I stumbled upon it, and if the OP is like me... he/she didn't replace the booster because nobody gave him/her a good reason to.

I had your same hiss for a few years. It eventually grew. It always annoyed me and my wife. Eventually, it got worse and the brake pedal would occasionally be hard (like you couldn't push it down, and thus couldn't brake for a few seconds). Shortly after that the car started throwing Check Engine error codes.

When Ski said "it's a PITA", he wasn't kidding. Supposedly you can access the nuts holding the booster by removing the wiper area cover, but that requires removing the wiper arms, and I wasted about 2 hours trying to do that before I gave up. Also, the how-to on this site says you can do it without removing the brake lines. That MIGHT be right, but it might also be wrong. They slightly changed the layout of that area of the engine bay through the years, and my 2003 ended up requiring me to undo the brake lines. Later, I found posts from another 2003 owner that found the same necessity.

In my case, I also "lost" my slow head gasket oil leak at the end of the brake booster replacement. I can't explain it, and neither can either of the two indy Saab mechanics I use for work that's over my head.

You can wait until you see other symptoms, or you can take care of it now. In my case though, it took the CEL and code, combined with the hard pedal before I realized what was going on. That sound drove me nuts for years up to that point though!

Good luck!
 

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OBD2 code P1576 by chance?

It's been quite a while since you posted your reply (to a thread that was old 6 years ago!), but I also just stumbled into it. I'm dealing with this issue now on an 01 9-5, almost exactly as everyone has described. No symptoms, except for the hissing when the brake pedal is released. I used a stethoscope to trace the vac leak to somewhere in the back of the booster tank.

Do you recall (by chance) what the code was? I realize this is probably testing your memory of an old problem.

The code I'm getting is p1576. I'm seeing different meanings all over. The list I found for Saabs says it's the brake pedal position sensor, and after testing with my meter I found one of the two to show inconsistencies. It's the switch that halts cruise control when the pedal is touched. Replacement is on the way.

But the first description (non-Saab specific) said it was the BBV Sensor Circuit High Voltage. I searched relentlessly for the sensor, and there's no mention of it anywhere on the interwebs for my car. Perhaps it's an internal sensor, built into the best? Shortly after I found the Saab OBD2 description of the BPPS. Now with the hissing, the non-saab code meaning seems more likely. But a mechanical switch is a helluva lot cheaper than a brake booster!

I'll be replacing the bb as soon as we're able to (since the at-home bb testing procedures all passed without a single problem), but in the meantime I was curious if the code you're referring to was the p1576?

Or perhaps another Saab enthusiast can verify? This is our first Saab, it was given to us last year. Lots of probs, and I'm trying to learn as much about my Saab as possible before dumping more cash into fixing it. My to-do list just keeps growing faster than I or our budget are prepared for...

Thanks!
 

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we had a hissing problem like this last year in a 2004 volvo and had the car at four different shops before a smart mechanic realized it was just a $10 one way valve. not sure if SAABs have a similar set up, but before we replaced the $10 valve, $800 worth of other work was done unnecessarily. just in case this MIGHT be able to save you similar trouble and money, i'm sharing it.

in the case of the other swedish car, the brake pedal began to sink after the car had already come to a stop.

it was the one way valve near the master cylinder that takes vacuum from the engine manifold. one shop had thought it was a bad brake booster. one thought it was a bad master cylinder.

this is the valve (which i know is used at least in some german cars too):
https://www.ebay.com/itm/Ate-34331151532-Power-Brake-Booster-Check-Valve-/122633910879

good luck...
 

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Myelectronsbiteback,

VolvoP1900 identified something to check - your brake booster vacuum hose and grommet between it and the booster.

I see you have a 2001, but in this aspect all 9-5's are pretty much the same, except for differences between Manual and Auto transmission cars. The auto trans cars have a "Y" semi-rigid pipe with two check valves. The manual trans cars have a pipe with only one. The usual problem is that the pipe (hose ?) gets more brittle with age, and it breaks. But P1576 is not the most usual code that owners see from a broken pip.

Regarding the brake pedal switches, you can likely get by with a removal, cleaning, and re-installation. Be careful, because one of the Facet switches that is sold as a SAAB replacement works just backward of how it should (ask how I know).

With the hiss you describe, I would look really hard at the GROMMET on the brake booster where the vacuum connection is made. If it's the problem, this would be your $2 fix.

AND one last thing, is a comment from another board by a long time SAAB mechanic (LINK). He mentions a brake light wiring short.
 

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In the RHD even I had my whole engine out and sitting in the bay, and the black cover off the top of firewall and windscreen I could not access two of the three bolts of the booster. I dread that job if have to do. It seems access is easier on a LHD.
 

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Andy,

If you ever have to replace the booster, check out how the bracket with those unreachable nuts is fastened to the firewall on your RHD 9-5.

I don't know quite what the RHD cars are like, but removing the booster on the LHD 1999 SE that I owned went pretty well by removing the bracket WITH the booster. The bracket is held on by three bolts and two of them are reached by long extension and sockets from under the car. IMO it's the better way to do this job. I first found that method posted by someone in the Netherlands, and I edited and added detail back when I posted my experience on another board.
 
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