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Brake pedal firm when engine not running..goes to floor when running! Forums > > Brake pedal firm when engine not running..goes to floor when running! Brake pedal firm when engine not running..goes to floor when running!
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Frontier Nissan Frontier/HB/Truck Discussion Forum

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  #1  
Old 04-14-2010, 12:09 PM
ronald467
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Default Brake pedal firm when engine not running..goes to floor when running!

I have a nice firm brake pedal when the car is not running, as soon as I start the engine, the pedal goes almost to floor.

So I ruled out the master cylinder, since I just replaced it last week. I bled the brakes and the master cylinder at least 10 times.

The diagnostic check on the brake booster appears normal, except that when I start the engine with my foot on the brake, the pedal goes almost to the floor with a very mushy pedal.

Can somebody shed some light on this problem? The brake booster holds vacuum just fine so again I am lost. What could this problem be?

Anyone have any ideas?

99 Nissan Frontier 4x4 (ABS bypassed) V6 - New Calipers, New pads, new brake lines, factory rebuilt master cylinder.
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  #2  
Old 05-27-2010, 04:13 PM
metal_clutch
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It's either a vacuum leak or your booster is leaking. Booster runs off vacuum. Booster run brake systems usually require replacing the booster and master cylinder at the same time. What typically happens is the master cylinder starts leaking and consequently leaks brake fluid into the booster. This destroys the seals within the booster and you loose pressure. In a slight leak, the booster pressure can be built up when the engine is off, because it's not pulling vacuum. Once you crank it up, your pedal hits the floor.

Either that or you still have air in the brake system. Do you have to purge an ABS system on these vehicles when bleeding the brakes?
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  #3  
Old 05-27-2010, 05:30 PM
ronald467
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I replaced everything with a good used system and re-bled the brake system another 10 times and I now have brakes, I really don't know what the problem was, probably a little bit of all of the above. At least now I have brakes, and Mr. Whizard fixed my entire dash cluster problem so I am a happy camper now.
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  #4  
Old 05-28-2010, 12:05 PM
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jlg2002 jlg2002 is offline
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Quote:
Originally Posted by metal_clutch View Post
It's either a vacuum leak or your booster is leaking. Booster runs off vacuum. Booster run brake systems usually require replacing the booster and master cylinder at the same time. What typically happens is the master cylinder starts leaking and consequently leaks brake fluid into the booster. This destroys the seals within the booster and you loose pressure. In a slight leak, the booster pressure can be built up when the engine is off, because it's not pulling vacuum. Once you crank it up, your pedal hits the floor.

Either that or you still have air in the brake system. Do you have to purge an ABS system on these vehicles when bleeding the brakes?
Where the Heck did you derive this off the cuff booster comment from? I've been a ASE certified brake technician and this has never been the case with automotive vacuum assist systems
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  #5  
Old 05-28-2010, 12:56 PM
jacko48j jacko48j is offline
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Good question jlg. Some of the "help" that shows up here from time to time has me asking the same question. There is alot of good help here but sometimes------not. Thankfully it is not very often.
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  #6  
Old 05-31-2010, 03:21 AM
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Does this mean that when i'am in park with the engine running the block rotates around the crankshaft? I can't wait to hear how an a/c system works...
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  #7  
Old 07-25-2019, 11:59 AM
TomL77 TomL77 is offline
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Did you ever solve this problem. I am having the same problem with my 2010 Frontier. Just replaced the MC, pads and all hoses. Still having the same problem. I had a 2004 Frontier and the brakes were outstanding. I would expect the same on this newer one. Truck has 102K miles on it. I purchased it with 90K. It stops OK but peddle is squashy.
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  #8  
Old 07-28-2019, 08:12 AM
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smj999smj smj999smj is offline
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Did you bench bleed the master cylinder before bleeding the system? If you did..good. If you bled your brakes the ol' "bleed from the furthest brake up" method, that may be the problem. Nissan has several different brake bleeding procedures depending on the year and model. Always refer to the factory service manual procedure before you start bleeding the brakes. If you don't have an FSM, NICO Club's website has free online manuals available for use.
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  #9  
Old 07-28-2019, 10:32 AM
TomL77 TomL77 is offline
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Thanks for the reply. Yes I did bench blead the MC. After replacing the hoses I did blead the system. I just blead each one as I replaced the hoses. Both front first then both rear. I did not refer to the manual which I do have. I will investigate. One thing that I did noticed while observing the MC and boaster while a friend pressed hard on the peddle with engine running is that the MC moves slightly forward with the surface of the boaster. Could it be that Nissan is making the boasters from thinner material. My wife has a 1014 Rogue and it has a firm peddle always.
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  #10  
Old 08-03-2019, 08:56 PM
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Quote:
Originally Posted by TomL77 View Post
Thanks for the reply. Yes I did bench blead the MC. After replacing the hoses I did blead the system. I just blead each one as I replaced the hoses. Both front first then both rear. I did not refer to the manual which I do have. I will investigate. One thing that I did noticed while observing the MC and boaster while a friend pressed hard on the peddle with engine running is that the MC moves slightly forward with the surface of the boaster. Could it be that Nissan is making the boasters from thinner material. My wife has a 1014 Rogue and it has a firm peddle always.
Are you sure it's not the firewall that's flexing? Just a thought. I remember working on an old Datsun 720 that had a weak firewall due to corrosion and the brake booster would move forward when you pushed the brake pedal down with force. Otherwise, Gen 1 Frontiers typically have solid-feeling brakes when they are properly adjusted. There are a number of people that complain that they grab too hard or lock-up.
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