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Forums > > Brake pedal firm when engine not running..goes to floor when running!
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Old 04-14-2010, 12:09 PM
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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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Old 05-27-2010, 04:13 PM
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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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Old 05-27-2010, 05:30 PM
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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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Old 05-28-2010, 12:05 PM
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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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Old 05-28-2010, 12:56 PM
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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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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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