funny air sound coming from brake area

alright, since about a couple weeks ago I've had this weird/funny sound like air escaping out of a minor leak in an oxygen tank, that faint fffffff sound. it seems ot be coming from my brake pedal area, cause whenever it starts up I just push the brake down and it goes away,b ut will eventually return.

Can anyone provide me some help please?

Thanks,
Jimmy

edit: thought I'd add specs- 99' limited edition, V8 4.7

I have the same thing happening in my 94 ZJ, I am not sure what is causing it, I never saw it as a problem - so I never posted about it, it's been there ever since I had the jeep (since febuary), but if it is a problem then someone on here will know the answer.

sounds like the abs going through its diagnostic cycle. Its supposed to do this every startup, but usually sounds like more of a hum than a leaking tyre.

well mines a swishing sounds in the pedal everyime I push on it. like air escaping

yea mines swishing too

Ok, what it sounds like to me is the same problem I had. I have a 93 zj, and it did the same thing for a long time. Eventually and periodically my brake pedal would feel squishy but would return to normal. I also heard the quite hiss as I first applied the brakes. But finally the problem showed its true nature, as the brake booster seal was leaking and finally failed. Making a sound like an air hose leaking everytime I pushed on the brake, leaving me with very little ability to stop. Start your jeep and bend down under the dash. Take a flashlight and look at the fire wall entrance of the brake pedal. There should be a ruber seal with asmetrical cut outs. Listen very carefully as you easily push on the brake pedal. If the sound is coming from this area, your brake booster is leaking ("internal" seal). This is not repairable and the unit will need replacment. AS the problem does progress, you should also notice a drop or a rough idle when you are pushing on the brake pedal. This is do to the use of postive manifold pressure. In the engine comparment you should see a single hose that leads from the intake manifold to the lower passenger side of the brake booster. Check this for leaks also. Brakes are the most important thing on a vehicle and a delay in a repair will hurt you or others( as I'm sure you are aware) hope I have been of help

As gunjes says there is a possibility that the booster is bad but>>>>>>>>>>>>>>

The area around the booster pushrod from the brake pedal is the intake for the atmospheric air pressure that helps apply the brakes. Look closely and you will see a foam packing around the pushrod.

You will hear a swish as the pedal is moved. When the pedal is held down in a fixed position the swish should go away. Likewise when the pedal is released and all the way up.

If the swish of air happens when the pedal is held down or released, then you have a problem.

The operational test for a vacuum booster is:

1. With engine off, pump the brakes 4-5 times. - this will remove any vacuum stored in the system.

2. Step firmly on the brake pedal and start the engine...

3. the pedal should fall slightly under you foot. - This tells you that the engine has removed the air from one side of the diaphram in the booster and you have atmospheric pressure on the otherside to help you apply the brake.

After the engine has run for a few minutes, a reserve of vacuum should have built up in the booster and some times in a separate reservoir canister.

Shut the engine off and listen for that hissing sound of a leak around the brake pedal push rod. Leave the vehicle sit for one hour. You should be able to fee atleast one boosted brake application when you step on the pedal after that time.

JD