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Old 09-15-2007, 10:49 PM   #1
tedconnor
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Brake light comes on and goes out.

I have a 1999 Nissan Quest. When I accelerate the van the light comes on and then goes out. When I brake the lights stays on for 5 seconds and goes out. I checked the brake fluid level and that is fine. The emergency brake is also fine. It sounds like the float in the master cylinder. Is there anything that I can do to fix the problem? Any additive that you can add?
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Old 09-18-2007, 04:58 PM   #2
GRNDSM
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Re: Brake light comes on and goes out.

Quote:
Originally Posted by tedconnor
I have a 1999 Nissan Quest. When I accelerate the van the light comes on and then goes out. When I brake the lights stays on for 5 seconds and goes out. I checked the brake fluid level and that is fine. The emergency brake is also fine. It sounds like the float in the master cylinder. Is there anything that I can do to fix the problem? Any additive that you can add?
How are the pads on your van? Our '04 did the same when the pads were fairly worn (down to 20% or so). Changing the pads/rotors solved it. There were no sensors on the pads to trigger the light and we still had plenty of fluid, so I am guessing this was done through ABS computer.
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Old 01-16-2008, 09:17 PM   #3
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Re: Brake light comes on and goes out.

I know this is an old post but somebody may be looking for the right answer one day.

There is no electronic sensor on the pads, the only sensors are the metal sensors on the pad backing that scrape the rotor when the pads are too low, but that has nothing to do with the light, direclty, and the ABS module has nothing to do with it either. It's actually a very simple and basic system.

When your pads wear down, the piston in the caliper needs to be extended more to keep the pads right next to the rotor, that's why you can't feel your brakes getting low, like in the old days with drums, unless you still have rear drums, then you'll feel it when they wear, but the pad never has to travel more like drum shoes do when they wear down. In order to keep that piston or pistons extended further out, more brake fluid has to fill the bore or cylinder behind it, which means more fluid is out in the system and not in the master cylinder.

When there is just enough fluid missing from the master cylinder, the sensor in the resevoir detects it and flips the light on. That's why when it first starts the light tends to go on and off. When you step on the brake more fluid goes out into the system and at the same time the fluid in the resevoir swishes forward, both of these things drop the level just enough and trigger the light.

When you let off, fluid returns to the master cylinder and the momentum of the vehicle changes and the float switch opens again, turning the light off.

When you replace the pads, you or whoever does the job, pushes the caliper pistons back in before seating the new pads in the caliper, so all of the fluid behind them goes back through the lines and into the master cylinder again, thus raising the level and keeping the switch open and light off.

This is the main reason you should never add brake fluid, if the level is low, most likely it's out in the brake system and you are going to need brakes. Unless you have a problem it's not going to leak out, it's a sealed system, and if it is leaking you will know it before that light comes on. It's not like the engine oil, that can burn off or slightly leak from a few places, so you need to replace it, you generally will never have a brake fluid leak unless something is very wrong.

If you add fluid, then whoever does your brakes will have an overflow mess when they push your caliper pistons back in, believe me it's happened to me plenty of times and it's a mess, not to mention brake fluid will strip the paint right off your firewall, so it's not a good idea to ever add any.

Even if your fluid level looks OK, it is probably just at the right level to close the switch intermitently and turn you light on.

20% off pad left sounds about right to see an intermitent brake light on.

Hope this helps somebody doing a search at some point.

Later.
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Old 01-17-2008, 12:01 AM   #4
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Re: Brake light comes on and goes out.

mmcpeck,

I know exactly what you are describing here. And I completely agree that people should NOT add extra fluid!

But I still think that there might have something to do with the ABS computer!

When I say "we still had plenty of fluid", I mean it:

- I opened the brake fluid reservoir and made sure that I was seeing an actual level (sometimes those plastic reservoirs are difficult to look through and register an accurate reading). It was still between the two marks!

- Then, I made sure that the float wasn't stuck in the bottom, as I have seen on some cars in the past. It was still floating at the surface.

- Then I physically lifted it, as high as it could go and the light remained on.

Now, unless there is some built-in delay in the circuit, I am pretty sure that it wasn't the fluid level that caused the light.
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Old 01-17-2008, 08:36 PM   #5
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Re: Brake light comes on and goes out.

Well you already proved that fluid was the cause, because you said that replacing the pads solved the problem, when you did that all of the fluid was pushed back into the master cylinder. If you had a master cylinder full of fluid as you say, then you would have had an huge overflow when you put those new pads in.

I have worked on cars professionally for 15 years, 4 of them for Nissan, and trust me when I tell you that the ABS module has nothing to do with that light. If you had a problem with your ABS system, then you have a separate light on the dash for that. And you never would have fixed and ABS problem by just putting new pads on. The ABS module monitors wheel speed at all 4 wheels and tells the pump when to kick on, and which whell to reduced fluid pressure to, that's it. It has no idea what condition the pads are in.

Let me just ask also, that when you checked the float and lifted it as high as it would go, was the car running or ignition just on, and if it was running was the parking brake on? If only the ignition was on then the brake light will stay on, and obviously if the parking brake was on while you were checking this, then your brake light would stay on also.
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Old 10-26-2014, 11:58 AM   #6
mke039
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Re: Brake light comes on and goes out.

had the same problem. Brake warning light was on after doing brake job.

Saw the float in master cylinder reservoir tank was stuck.

Put more fluid in, saw float come loose and float...

Viola! Light is off
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