Does your brake pedal hiss at you?
 

One thing that annoys me on my V40 is that every time I press the brake pedal it makes a hissing sound inside, exactly as if I'm pressing on a damn foot pump or something. It can also be heard with the bonnet up, near the vacuum booster. The pedal is also kind of spongy, top part of the pedal travel gives little braking. And this sponginess corresponds with the noise, noise occurs in the weak part. All of this is only when the engine is on and the power brakes are working, when the engine is off the pedal hardens up properly after a couple of pumps (also accompanied by hisses). No hissing when pedal returns as well (though a little different more silent noise does exist, as if it's sucking), only during the press in. I bled the lines recently along with a brake fluid change and nothing changed, really.

So I've been wondering, is this just the way the power brake system is designed or is my vacuum booster diaphragm leaky or something? Is your pedal rock hard?

Unless you have snakes in the footwell, something is leaking. Try getting someone to pump the brakes whilst you listen to the connections from the vacuum system to the master cylinder. Master cylinder faults are rare and the V40 has typical saloon car feel to its brakes. Wild guess but a small leak in a vacuum line connection to the booster would be my guess.

If there was a vacuum leak I'd think the hiss would be constant and probably accompanied by a hardened brake pedal. In my case the hiss occurs only when the pedal is traveling down and the pedal is squishy. I'll have to listen where exactly the source is, if you say that there shouldn't be any noise.

Umm, it does not sound like booster trouble as the pedal would go hard, but the amount of vacuum used will vary, which is why it sounds like a small intermittent leak. Post a recording to You tube, just in case it's a turbo fault plus incorrect bleeding.

More info about hissing brakes:
https://www.yourmechanic.com/service...sed-inspection

Hows your cruse? There is a vacuum system there as well. I think you would have noticed the brakes failing or "fading" but the cruse maybe not.

Found this thread and the 2nd post shows a photo of the peddle box whith the vacuum thingy:thumbs_up:
http://www.volvoforums.org.uk/showthread.php?t=179017

I wouldn't call it intermittent, as it occurs reliably and every time I press the brake pedal, as long as the engine is running. It's most definitely got nothing to do with the turbo, sounds like a loud and clear whoosh whenever I press the pedal. Exactly like a foot pump. I can try to record the sound if it helps.

The link you provided mostly addresses constant hissing, which would definitely indicate vacuum leaking. The "damaged foam silencer" could be relevant, though. As I understand most power brake systems release air from the vacuum booster whenever the pedal is pressed. Though I haven't ever heard it being so loud on any other car, there is usually a foam filter/silencer to mute that, maybe it's missing. The spongy pedal doesn't help as well, not sure if it's related, though.

Cruse, cruse... cruise control? Non-existent, unfortunately. Though I'll check if the vacuum thing is there.

The pneumatic sound is perfectly normal, stop listening for it and it will go away or take off the exhaust.

The web site is advertising a service, they want your money
however at least they they have included this

THE HISSING NOISE YOU HEAR WHEN THE BRAKE PEDAL IS DEPRESSED COULD BE NOTHING MORE THAN ORDINARY OPERATION

COULD, to maintain your interest in their service

You have included all this in your post

"The link you provided MOSTLY ADRESSES CONSTANT HISSING, which would definitely indicate vacuum leaking. The "damaged foam silencer" could be relevant, though. As I understand most power brake systems release air from the vacuum booster whenever the pedal is pressed. Though I haven't ever heard it being so loud on any other car, there is usually a foam filter/silencer to mute that, maybe it's missing. The spongy pedal doesn't help as well, not sure if it's related, though."

Completely omitting the ORDINARY OPERATION bit.
Just as they designed

The only time you're going to have a hard pedal with these systems is when the car is turned off.

Never heard any type of hiss from my brakes and no one else has mentioned hearing such a noise. Perhaps the OP has a phase one with different sound insulation, or even a different engine.

I call it a brief pneumatic sound and then it is supposed to stop
when the pressure on either side of the diaphragm is equalised.

May be a torn diaphragm but that's just inventing problems where none exist

I like your insulation, or a rotten silencer, idea.

I've got hissing and there's definitely zero problem with my brakes
as many a pushy pedestrian will attest