Question about 1990 Volvo 740

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When you press on the brake it sounds like Air

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Your vehicle has what is called a brake booster on it. It is mounted on the firewall under the hood and your brake master cylinder is mounted to that. The brake booster works off of vacum pressure that your engine builds. When the booster goes bad the vacum pressure escapes and it sounds like a hiss at your pedal. This problem will get worse and can cause your pedal to go to the floor. This could be a dangerous situation for you. I recommend that you get this checked out for safety reasons. Hope this helps. If for some reason this goes out on you and you are on the road you can find the big vacum hose that goes into the booster, take the hose off and plug it with something. Please note if you do this you will not have power brakes after that you will be on manual brakes and it takes a lot more pressure to stop your vehicle so drive slowly and not to close until you get it fixed. If the booster goes all the way out your engine will run rough or die at idle. If you do have to block the vacum line your engine will run smooth again but it may set off the check engine light. Take care, Roger

Posted on Feb 07, 2010

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1 Answer

When I press the break and then release there is an air leak sound. What is the cause?


The brake servo or booster is divided into two chambers separated by a flexible diaphragm. Briefly there is vacuum on one side and pressing the brake pedal allows air to enter the other side and this is how the brake pedal effort is boosted.

The air enters via a filter through which the pushrod connected to the brake pedal passes. When the brake is applied the hiss of air entering the chamber will be audible; more on some models than others.
When the forces are balanced; when the brake is sufficiently applied no more air will be required and the hiss will cease. When the brakes are at rest; not applied there should be no hissing.

If there is it indicates a problem with the brake booster, perhaps dirt in the valve gear. Although repairs are possible they aren't very practical to carry out. A repair shop would probably charge more for fitting an overhaul kit than for replacing the unit.

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Brake pedal drops very low on first press only


It is most likely you have air in the brake fluid. The first press of the pedal compresses the air and the effect lasts for a little while.
Another cause may be warped (bent) rotors, the pads can be pushed back into the calipers by the off center area of the rotor. This means that the pads have to travel further in order to contact the rotors when you press the pedal.

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1997 Ford Thunderbird, Brakes make a hissing sound when pressed


Sounds like the vacuum booster.Check all hoses to and from it

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96' Chevy K1500 Brakes are hard to press, but u hear a lot of air coming out, if i hold the brakes down for too long the truck cuts off.


It sounds like a problem with the brake booster, the air diaphragm that gives you power brakes. It is located right behind the brake master cylinder and mounted to the firewall.
Check the large vacuum hose to it for leaks. If the air is coming out of the unit itself, it will need replacing.
It will stop the engine because with a large air leak, the engine just sucks in way too much air. Hard brakes are also an indication that the booster is not working right. But check the hose first for leaking.
Good luck.

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I have a 2004 dodge dakota my brakes work but when i press them they make a sound like they are releasing air


there is no problem the master cylinder is connected to a vacuum device called a brake booster when you press brakes it sucks air out of it it is the big black round thing connected to master cylinder which is the metal thing under brake fluid holder

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Why do i hear air when i press the brake pedal


well, you don't really hear air when you press the brake pedal. You might hear hydraulic fluid going through the valves in the master cylinder, that sounds like air flowing through a vent. But, I assure you, it is not air.

One can press and pump on the pedal when the car is off and hear the 'rush' of fluid which sounds kind of like air rushing out of an inner tube.

It's a common mistake to believe a rush of air is being heard when pushing on the brake pedal.

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2001 DeVille: Cad 2001 deville, When I press the brake peddle, I...


thanks for the good information

the air sound AND the hard pedal means you have a brake booster concern

if the car is running ok we can rule out vacuum leaks so that leaves the check valve but with the car running even a check valve wont cause the concern

therefore you need to replace your booster

semi-tough job because you have to remove the master cylinder and typically there may be some push rod adjustments afterward

depending on your level of experience you may want to seek professional assistance



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1998 Suburan brake pedal gushes air along the firewall.


you got it, the booster is leaking reducing the vacuum to the intake, replace it

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A sound like that that you can hear inside has to be the vacumn assist tank leaking air. Did you bleed the new master cyl ?

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