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Discussion Starter #1
Hey guys how you doing. So lately i've been driving on the freeway for about 30mins and i'll try to use my brakes but the pedal feels like it has lost all pressure. so it slowly goes almost all the way down before i feel my brakes actually working. its pretty scary because when i really need to stop my brakes are going to be delayed. Now this only happens when im either on the freeway for about 20mins in 5th gear or im driving without using my brakes for an extended amount of time. Once i try to use my brakes and i feel they have no pressure all i have to do is pump my pedal and pressure comes back like normal. Im just wondering what could be causing this.

thanks for all the help guys, im driving a 98 sentra 1.6 ga16de with 130kmiles
 

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If your brake fluid level isn't dropping[ leaking brake piston seals ], most likely the diaphram in the brake booster is torn, so the vacuum is lost and no boost. To check the booster , with engine off put foot hard on brake pedal then start engine, if booster is good pedal will sink down towards firewall. If the diapram is bad you will need a new booster fitted. The reason it happens on the freeway is there is less vacuum supply at speed so you have less to supply the leaking booster, around town supply must be meeting demand although your brakes in town are probably not as good as they could be either.
 

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I would think if he was pumping the brakes to get them to work that it wouldn't point to a torn diaphragm since the act of pumping the brakes effectively bleeds out any vacuum stored in the booster itself.
And, yes at highway speeds, there is less vacuum pressure available, but there is also more vacuum volume available. And if you're on the highway, practically the last thing you do before hitting the brakes is lift off the gas pedal, which in turn raises the vacuum pressure, even if for a second or two, which should 'refill' the booster almost instantly.
I would lean more towards something like the fact that the fluid might've gotten low at one time or another and now there's a bunch of air in the lines somewhere and the brakes need to be bled out.
The other case I'd lean towards is a bad primary piston seal in the master cylinder...which will cause the pedal to fall until it's 'tagged' once or twice, enough to get the o-ring to flare out a bit and make a good seal.

Start the engine, let it idle for a minute or two, shut the engine off, let it sit for a few minutes. Do you hear a hissing coming from the brake booster? If so, is it coming from inside the booster or from the hose going into the booster? If it's inside, you need a new booster, no question. If it's the hose, you might have a bad check valve.
After a few minutes (5-10-15, whatever) have gone by, get in the car and step on the brakes (full stroke) a few times. Does the brake pedal get progressively harder or is it hard from the first push? If it gets harder and harder with each push, the brake booster is likely good, but you might have air in the lines or have a bad primary piston seal (i.e. need new master cylinder).
After you've pumped the brake pedal a dozen times or so until it's good and hard, let it sit for a few more minutes (5-10 whatever), then push on the pedal fairly hard and hold it. If it drops to a certain point and sits there, then the master cylinder is likely good. If it drops to a certain point and then slowly drops to the floor, the primary piston seal in your master cylinder is likely bad and you'll need that new master cylinder. If it sinks to the floor on the first push, you've probably got air in the lines and need to bleed the brakes completely.
Got all that? My fingers are pooped...
 

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Discussion Starter #4
lol well i think im gunna go read up a bit on the brake system so i can better diagnose it and better understand you guys. i will let you guys know what happens. i will definitely go try out those things you listed. thanks a lot! :)
 

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If your brake fluid level isn't dropping[ leaking brake piston seals ], most likely the diaphram in the brake booster is torn, so the vacuum is lost and no boost. To check the booster , with engine off put foot hard on brake pedal then start engine, if booster is good pedal will sink down towards firewall. If the diapram is bad you will need a new booster fitted. The reason it happens on the freeway is there is less vacuum supply at speed so you have less to supply the leaking booster, around town supply must be meeting demand although your brakes in town are probably not as good as they could be either.
disagree. booster failures cause hard pedal, not super soft. he's describing a spongy or soft pedal.

for soft pedal, bleed the brakes. you either have dirty/boiled brake fluid, or air in the brakes. bleed the system, and hopefully it fixes the problem. also, it's a very cheap fix and a regular maintenance item, so if it doesn't work, it's still a good thing to do anyway.
 

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disagree. booster failures cause hard pedal, not super soft. he's describing a spongy or soft pedal.

for soft pedal, bleed the brakes. you either have dirty/boiled brake fluid, or air in the brakes. bleed the system, and hopefully it fixes the problem. also, it's a very cheap fix and a regular maintenance item, so if it doesn't work, it's still a good thing to do anyway.
Isn't that what I just said in #3? :eek:
 

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Discussion Starter #8
ok cool. so i'll bleed my brakes. i've never done that before so could someone post a quick guide on that? and what type of brake fluid i'll need. thanks :)
 

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Best to get a workshop manual or general purpose brake manual to follow. Check your owners manual for correct fluid. Most likely DOT4 brake fluid, but ask at store if not sure on type.
 

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Long pedal travel

I know this is a very old post but I thought it may help someone that is doing a search for the same problem.
I have an 09 Murano and was perplexed why the pedal felt like I had air when I hadn't disconnected anything in the hydraulic system. I had all the brake pads replaced 6 months ago and confirmed that they were still 50%.
I started on line searching for the problem and everyone started talking about master cylinders and boosters. I thought it didn't make sense that these parts would be gone on a car I felt was still fairly new. I also confirmed no leaks or dampness anywhere. I had 10yr old cars that never needed these brake parts that people were talking about.
Anyway on the weekend I decided to take apart the front brakes and troubleshoot.
I soon discovered that the slides were seized in the out position on both sides. I didn't think that this would happen so soon after having the brakes serviced 6 mnths ago.
After I freed them up and greased the metal parts, everything works great and my pedal returned to top high. No bleeding necessary.
FYI
 
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