* Vacuum leak. A vacuum leak can cause a hissing sound under the dash when you brake because the vacuum booster that assists with braking is no longer able to function properly. This can be caused by a damaged vacuum hose or a faulty vacuum check valve.
* Brake fluid leak. A brake fluid leak can also cause a hissing sound under the dash when you brake because the brake fluid is leaking out and the air is being sucked in. This can be a serious safety issue, so it's important to have it checked out by a qualified mechanic as soon as possible.
* Faulty brake booster. A faulty brake booster can also cause a hissing sound under the dash when you brake. The brake booster is responsible for amplifying the force you apply to the brake pedal, so if it's not working properly, you'll have to press the pedal harder to stop the car. This can also be a safety issue, so it's important to have it checked out by a qualified mechanic as soon as possible.
If you're experiencing a hissing sound under the dash of your Volvo after braking, it's important to have it checked out by a qualified mechanic as soon as possible to identify the cause and get it repaired.
