Hissing noise from brake booster

[Hazwan]

Theres a hissing noise from my brake booster with the engine on and continues after few seconds turning the engine off. My engine idles perfectly fine and my brake pedal is fine as well. In fact I don't notice anything strange with my brake feel or how my engine runs. Dying brake booster? I'm guessing its a small leak since I didn't notice anything strange?

[A18A]

im guessing a small leak too

[2oodoor]

If it is hissing you do have a vacuum leak, and yes it will idle even better if you fix it!
Where is it hissing from exactly in relation to the booster, inside the car? around the vacuum check valve? The check valve can be replaced without doing the whole booster.
Sometimes your master cyl can leak brake fluid into the booster, too so if you wind up replacing, do both of them.
Boosters do use air from the vaccum and moisture comes in along with the air and over time it will rust out the inside of the booster.

[P|eszczoH]

I dont want to make a new thread so...
when i'm braking I can brake preatty well but if i want I'm able to put my braking pedal to the metal.
when i take my foot of it i hear strange sound, like sucking for a moment...
I thought i have to change my brake pump, but this post cncerned me a bit...
maybe my brake booster is giving up?
when i'm starting my engine the rpm goes down for a while and after that they go up to normal rpm, and it doent happen when there is cold couse choke comes into action and raise them up.
Should I be concerned about my brake booster ?

ps. no leaks of brake fluid, done brake calippers a year ago and all rubbers, I thought the pump gives up beetween front/rear section.

is there a way to check the brake booster ?

[thegreatdane]

sounds like either air in the brake system, try bleeding it properly and use new fluid, or it could be the master cylinder not sealing properly making the brake fluid pass by the piston(s) thus the pedal going to the floor.

[Hazwan]

If it is hissing you do have a vacuum leak, and yes it will idle even better if you fix it!
Where is it hissing from exactly in relation to the booster, inside the car? around the vacuum check valve? The check valve can be replaced without doing the whole booster.
Sometimes your master cyl can leak brake fluid into the booster, too so if you wind up replacing, do both of them.
Boosters do use air from the vaccum and moisture comes in along with the air and over time it will rust out the inside of the booster.

No noise from the inside. I'm not sure which part is hissing, I'll check it today. I really hope that I don't need to replace the whole booster.

[P|eszczoH]

sounds like either air in the brake system, try bleeding it properly and use new fluid, or it could be the master cylinder not sealing properly making the brake fluid pass by the piston(s) thus the pedal going to the floor.

done year ago with calliper rubbers and so on...
also if i put the brake pedal to the floor the rpm goes down like about 500rpm...
I think that brake pump is flushing between sections,
hope that brake booster is good, to check it takes gauges that i do not have

[noidea24]

Sorry, everyone, to bring up a post from 2007.

But this link popped up in Google while running some search queries, and that means more people than just me have searched it before.

I just want to say that there is an easy way to diagnose your brake booster without gauges and such.

Stolen from hondatech.

Functional Test

1. With the engine off, press the brake pedal several times to deplete the vacuum reservoir, then firmly press-and-hold the brake pedal for 15 seconds. If the brake pedal sinks, either the MC is internally bypassing, or there is a leak in the brake system. (mc, lines, prop valve, calipers, wheel cylinders, etc)

2. With the brake pedal pressed, start the engine. If the pedal sinks slightly, the vacuum booster is okay. However, if the brake pedal height does not vary, the check valve or booster is faulty.

3. With the engine running, lightly press the brake pedal. If the pedal sinks more than 3/8" (10mm) in three minutes, the master cylinder is faulty. (If the A/C is on, a slight change in the height of the brake pedal is normal.)





Leak Test

1. With the engine running, press-and-hold the brake pedal, then turn the engine off. If the pedal height does not vary while pressed for 30 seconds, the vacuum booster is okay. If the brake pedal rises, the booster is bad.

2. Turn the engine off and wait for about 30 seconds. Press the brake pedal several times using normal pressure. When the brake pedal is first pressed, it should be low, then gradually rise with each consecutive compression. If the position of the pedal does not vary, inspect the booster check valve.

3. Disconnect the vacuum hose (check valve built-in) at the brake booster.

4. Start the engine and let it idle. There should be vacuum available. If there is no vacuum, the check valve is not functioning properly. Replace the vacuum hose, and retest.

5. With the engine running, pinch the vacuum hose between the check valve and the booster.

6. Turn the engine off and wait for about 30 seconds. Press the brake pedal several times using normal pressure. Again, the brake pedal should be low when it's first pressed, gradually rising with each consecutive compression. If the pedal position does not vary, replace the booster. If the pedal position does vary, replace the vacuum hose / check valve assembly