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Sounds like the vacuum booster is leaking (it's that large round canister behind your brake master). You will have to locate the source of the leak; if it is the vacuum booster you will have to replace it. Do the brakes feel like they're not working? That you have to press on the pedal really hard to stop?
Check condition and operation of idle control valve. Also check condition of vacuum lines and brake vacuum assist hose and vacuum assist servo.
To check if you have a vacuum leak before the servo unit, first check hoses and connections, then pull off the vacuum hose to the servo unit and put your thumb on the end of it. You should feel a strong pull/vacuum. If you release your thumb then you should hear the engine revs climb, and then possibly go back to normal. If you feel lack of vacuum, then you likely have a leak somewhere.
Another thing to check is that when you pump the brakes several times, the engine note changes and your brakes become stiff for a second or two before becoming softer again. After you have run the engine for a few seconds and then turned it off you should be able to pump the brake pedal three times, with it being soft-ish each time until all vacuum has been spent from the servo unit, and then your brakes become stiff again. If this seems not to be the case, and they are still all the time, then you have a vacuum leak, possibly in the servo unit.
That is the sound made by a failed servo unit - it's used to assist the brake effort and it uses the vacuum from the intake manifold - you must replace it.
Since this is a critical component of the car and the slightest mistake could have severe results you should take it to a repair shop. It will cost you roughly $350: $100 for the part, $250 for the labor.
its condensation that is freezing up in the vacuum line that supplies the power brake booster.i have run into this before and if i remember correctly nissan manufactured a counter measure part for this complaint.contact local dealer parts dept. they should be familiar with this part.relatively simple to replace.
Try the following to confirm if is the servo unit .( engine off,pump few times the brake pedal to become stiff.Keeping pressed the pedal start the engine and if servo is working you will feel the pedal slightly goes down.If not will not move at all-remain stiff.) Also if you do have the check valve close to servo not working you can realize it by the engine idle ( erratic ) and the servo not working.The other possibility if the vehicle is fitted,the ABS unit itself.