Brake pedal goes all the way down, however it still brakes. - Nissan Forum
B11/B12/KN13 82-90 chassis B11 (1982-1985 Sentra), B12 (1986-1990 Sentra), and Pulsar

 
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post #1 of 14 Old May 26th, 2005, 11:21 PM Thread Starter
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Brake pedal goes all the way down, however it still brakes B12

1991 B12 Sentra ('87 equivalent to the U.S. I believe, carburated E16S)
Ok here is the situation:

I press on the brake pedal, it brakes fine for about 1 second, and then the pedal goes all the way down, but I can still brake, an observation I made is that if I press hard when the pedal is all the way down, the right front wheel will lock up.

Letting go off the pedal makes and pressing again restores the pressure which is lost again almost inmediately.

My mechanic told me it was "The pump" (I live in mexico, and he just tells me I need a new "bomba" which is a pump) but I fail to see where the hell does my braking system have a pump!

I have a service manual, and the closest to a pump is a valve that goes from the master cylinder to the intake manifold and I checked it and it works fine. What else could be wrong?

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Last edited by MomoB12; May 26th, 2005 at 11:41 PM.
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post #2 of 14 Old May 26th, 2005, 11:52 PM
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Numerous things can be at fault for the brake pedal going "all the way down," everything from a defective master cylinder to damaged brake lines.
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post #3 of 14 Old May 26th, 2005, 11:55 PM
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its more likely a seized front caliper or faulty rear drums(pads worn out, adjust nut not working properly, etc.)

i'd check them to see if they're working before buying new stuff.

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post #4 of 14 Old May 27th, 2005, 12:17 AM Thread Starter
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Sorry I forgot to be a little more specific.

There is no loss of braking fluid, and as I said I can still brake, however not as effectively. I'll check the front calipers, since the left front tyre is wobbly, so I guess my sis fell in some pothole or did something stupid and never told me.

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post #5 of 14 Old May 27th, 2005, 12:19 AM
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If the vacume side of your master cylinder is leaking you will loose your break assist and that can cause your pedal to go right to the floor, so maybe this is what your mechanic ment by pump. Just something to check.
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post #6 of 14 Old May 27th, 2005, 11:27 AM
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id say its the master cylinder.

Crazy !

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post #7 of 14 Old May 27th, 2005, 12:27 PM Thread Starter
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Quote:
Originally Posted by Craigpre1
If the vacume side of your master cylinder is leaking you will loose your break assist and that can cause your pedal to go right to the floor, so maybe this is what your mechanic ment by pump. Just something to check.
Well I just went to check that out and guess what... There does seem to be a leak there, since that park seems "humid". (See pic below)

Will changing the hose fix it? Or if the hose is ok, will re-adjusting the braces fix it?

Thanks!

Not a shadow, the line is darker because it's wet, surely braking fluid. Red wire is for my tach :P

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post #8 of 14 Old May 28th, 2005, 01:52 AM
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Quote:
Originally Posted by Crazy-Mart
id say its the master cylinder.

Ditto. If you push the pedal down and it brakes but the pedal starts to go to the floor and you can pump it to get pedal again then its going to be the master cylinder. a primary seal is leaking inside the master cylinder letting the pressure bleed off back to the reservoir which explains no loss of fluid.

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post #9 of 14 Old May 28th, 2005, 01:58 AM Thread Starter
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I see. Well tomorrow will be a long morning then, I shall dissasemble it and see what I can do. Thank god for this service manual, it has everything. (Actually thank xXB12RacerXx hehe)

EDIT: Oh btw, pressing on the brake produces this hissing sound on the pedal. Is this normal? It sounds a little louder than what I would consider normal.

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Last edited by MomoB12; May 28th, 2005 at 02:06 AM.
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post #10 of 14 Old May 28th, 2005, 02:56 AM
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The sound you are hearing is the vacuum being used up also you were given slightly off info. if you loose vacuum assist to your brake booster your pedal will be hard to push if the rebuild of the master cylinder did not fix it I would look at all your hoses you have 4 one at each wheel if they are buldging it can cause similar symptoms.

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post #11 of 14 Old May 28th, 2005, 03:20 AM
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I had a master go out and it had the same symptoms. As it was explained by a mechanic, I blew some kind of secondary seal. The master is designed that way for safety reasons.
The leak in the pic is not worth worry. It looks like normal seepage on an old car. If you want to replace the hose that would be ok. But don't go just moving it around.
Begin with a thourough inspection of all 4 brake slaves and their associated lines.
If you decide to go to Midas, walk in backwards and slightly bent over.
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post #12 of 14 Old May 28th, 2005, 01:46 PM
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Quote:
Originally Posted by Popkorn
If you decide to go to Midas, walk in backwards and slightly bent over.
might as well just put your hands onthe table and bite you tongue while you're at it.

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post #13 of 14 Old May 28th, 2005, 03:27 PM Thread Starter
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Just finished. I took it apart and well the seals are in good shape, no cracks no nothing. They're glued to the master cylinder too, however the glue does show some wear. Well I cleaned it all up, reassembled and took it for a test drive: No luck, the pedal went right to the bottom on the first braking, I pumped it and got some pedal, it didn't go down this time, I kept on going for about 100feet and braked again and it went all the way down again, after a few pumping and braking tries, the pedal is back to it's old self again: Brakes ok and then it goes all the way to the bottom.

Maybe the seals need to be glued completely again? I don't think the primary and second pistons need any checking right?
I did a brief check on my braking hoses and they seem ok, but later when the sun get's down I'll make a thorough check.

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post #14 of 14 Old May 28th, 2005, 10:01 PM
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then you should do a brake blead you may/more than likley do have air trapped in your system. Take the cover off of your master cylinder, open up the rear passenger wheel cylinder bleader don't crack the line there is a bleader on all wheels. let it drip till a steady stream of fluid comes out keep a constant eye on the fluid level in the resivour fill when needed if you want you can let it drip till clean fluid comes out then you know you have good fluid. When that one is done retighten the bleeder and move to the rear driver side and repeat then to the front passenger side and then the front driver side. This is a gravity bleed it works well but I would still recomend a traditional bleed but this should improve your pedal unless your master is just toast and you could not visually see it.

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There's no such thing as a soul. It's just something they made up to scare kids, like the boogeyman or Michael Jackson. -Bart Simpson.
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