Hear a Whooshing Sound When Stepping On the Brake Pedal?
When you apply the brakes, instead of braking smoothly, you hear a loud hissing sound. This unexpected noise indicates that there is a problem with the braking system.
Even if your vehicle brakes normally, it is dangerous to ignore this warning sign. Let’s take a look at what causes a hissing or whistling noise when you apply the brakes.
But what exactly is a hissing sound? The unpleasant noise starts subtly when you first press the brake pedal, as a quiet whoosh rather than the familiar friction of brake pads on brake discs. But as you continue driving, a louder, forceful, hollow sound develops, reminiscent of wind blowing through the mouth of a bottle.
The vague “whoosh” is noticeable when you apply the brakes with moderate pressure (and even more so at highway speeds).
At the same time, the brakes feel spongier, and more downward pedal travel is required to achieve the same reduction in speed. A loud noise that sounds like air is leaking from somewhere in the main brake system is bothersome. Brakes should not rumble or whistle. You should be able to clean and grind away components that are sure to slow your vehicle down.
Possible causes of whistling noises
1) Air in the brake lines
Air in the brake lines is often harmless at first, but if the whistling noise comes and goes, this could eventually be a cause for concern. Pumping the brakes can eventually force the air out of the lines, but in the meantime, your brakes may feel spongy.
However, air has gotten in for some reason, whether it’s a leak, low fluid level, or recent system maintenance. Moisture and impurities in the brake fluid can also get in and cause corrosion and malfunction over time. Adding brake fluid and bleeding the lines to remove the air pockets will solve the immediate problem, but be aware that performance will be compromised.
2) Worn brake pads
If you notice a decrease in stopping power, a loud squeal when braking, or increased pedal travel to a stop, your pads may be too thin to properly bond to the rotor. This creates space for air pockets between the components and causes the hissing noise.
If this is the case, it’s time to replace your brake pads. If the pads reach or exceed their minimum thickness, they can cause damage to the rotors and calipers that will be expensive to repair.
3) Vacuum Amplifier Leak
Brake boosters increase pedal force through engine vacuum. A cracked housing or defective internal diaphragm will prevent proper vacuum build-up to apply hydraulic pressure.
The brakes themselves rely on leg force, requiring a lot of effort and a sudden flow of air. A leaking master cylinder seal, hose, or fluid can also let air in and cause a hissing noise. Inspect these brake components and repair the source of any leaks immediately. Diagnosing the Root Cause
A thorough investigation of the details of the hissing sound and how it occurs is key to identifying the root cause.
Use the following test methods:
Visual Inspection
1. Check the brake fluid level in the master cylinder.
2. Check the brake calipers, brake lines, hoses, or areas under the vehicle for dirt or moisture that would indicate a leak.
3. Inspect the brake pads/rotors for excessive wear, cracks, and uneven patina that indicates overheating.
4. Note any cosmetic damage or rust on the brake components.
Test the response of the brake system
1. Pump the pedal to see if it feels spongy or sinks. This indicates air or fluid loss.
2. Does it take more pedal stroke to lock up? Do you need more pressure? This indicates worn brake pads.
3. Test with only the parking brake engaged to isolate noise from the hydraulic system.
Listen during operation
1. Have a passenger press the brake pedal firmly while listening under the hood to pinpoint its location.
2. Does the noise come from one area of the wheel or is it evenly distributed throughout the system?
3. Does the noise occur only when the pedal is partially or fully depressed? At a particular steering wheel position?
Evaluate performance changes
1. Does the vehicle have long braking distances?
2. Are the brakes uneven, pulling from side to side, or pulsating?
3. Is the hissing noise accompanied by a smell or sound?