Its a constent hissing from my brakes in my Mazda 2001 tribute. The noise does stop When i press on the brakes but as soon as i let off them the hissing continues. Ive read it could be the brake booster. How much if that What you think it sound like is that? And how Do u replace it? Than u so much fir your time. I really appricate it. Michelle barker

It is the booster, but it might not be abnormal.

Vacuum from the engine is supplied to the booster, and vented if the brakes are not being applied.

When you step on the pedal, the movement of the pedal blocks the vents, allowing the vacuum to build up. That vacuum then pulls on a diaphragm, assisting you in applying the brakes.

Most people can’t hear that venting, some can.

You should, of course, have it checked out, but it sounds as if this might be normal operation.

The booster is bolted to the firewall, and then the brake master cylinder is bolted to the forward side of the booster.

To replace it you disconnect the brake pedal, remove the master cylinder, take the vacuum line off the booster, unbolt it from the firewall.

Install the new booster by reversing these steps.

But I don't think you need one, from what you say.

It does sound like the brak booster. The change will take u the best part of the day.Remove the master cylinder and then the booster and unhook the breaking pedel from the booster.Install the new one and bleed breaks. Also get some DOT 3 breaking fluid.Hope this helps.

Check the brake booster by shutting the car off, waiting a few minutes, then pulling the thick tube with a check valve that leads to the firewall where the brake booster is mounted.

If you hear a loud hiss of air in the vacuum booster when it fills with air, then it's probably ok.

New nuts may have to be used to mount the master cylinder to the brake booster and the brake booster comes off of the firewall with a wrench.