2000 jeep grand ckee push on brake pedal and hissing noise gets loud release pedal and it gets quiter pedal does not get hard to push down but if held will slowly sink to floor

You have a leaking power brake vacuum (this is what you hear) operated power booster. Go to this link to read the procedure of removing and pacing the power brake vacuum booster

Sounds like your brake booster is leaking vaccum and will probably need to be replaced.

Is there any problems with the brakes besides the noise? If theyre acting up, it could be your brake booster. It's hard to determine without more specific details, brakes always make a quiet hissing noise, depends on where, how loud, how the pedal feels...

Hello moonc40: My name is Roger and I will help. With the engine off step on the brake a couple of times. Hold your foot one the brakes and start the engine. The pedal should drop a couple of inches. If it does not then your vacuum booster is most likely the problem. With out the booster assist your brake pedal will be extremely hard to push. If you hear a hissing sound inside of the vehicle when the brake pedal is pushed? The problem is the booster.Should you need further help please just ask. Please rate the answer this way I know it was helpful. Thank You for using Fix Ya. Roger

There are some solutions for this kind of problems:
1. change your brake fluid (BF). This need to done regularly every three yrs. , at hard condition's every two yrs.
2. check the caliper. It's possible that one of the two pistions inside the caliper don't work properly (mud or rust inside)
3. check if the brake bads can slip free over the anchor, if there are some rust on it, remove them and check the surface, if damaged, replace the part.
4. check the caliper slide pins. If the caliper can not slide, it will stay in contact with the brake disk, this will heat up the bad and disk and make a noise.
5. check your brake booster (vacuum test required). If you switch of the engine, listen if your hear some noises (air leak!).
If all that wont work, come back again.

Check the brake light switch, located under the dash above the brake pedal. The other noise may not even be related to brake lights staying on

it sounds like the diaphram in your brake booster is leaking.booster assembly needs replacement.

If you are talking about when you push the pedal you hear an air noise and the pedal is real hard to push then you most likely have a bad power booster. If your pedal is weak and goes low to the ground you most likely have a bad master cylider as well. Best to just change both. For that year model shouldnt cost much and not hard to install.

Sounds like it could well be the brake servo unit.

It it a brake booster problem.I am changing mine this weekend.You can get a new one form autozone for 149.00 or get one at the junkyard for about 70.00.I am installing mine this thanksgiving.I can send you the step by step instructions if you like.I have a 99 jeep grand cherokee limited edition and for once i thought i had a snake in the car..