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Old 11th December 2011, 01:54   #1
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Default Car moving faster on applying light brake

Hi all,

Took the car out for the company picnic today, Santro 2004 XS.

I think I hit some stones on the underbody on the ring road. Anyway the trouble starts after I have driven around 150km during the day. I apply the brakes and the car seems to speed up suddenly like I pressed the clutch and car became free.

This happened a few times, stopped the car, called a mechanic who checked the underside and brakes under the wheels. All seemed fine and the trouble disappeared. Dont know what caused it but am scared that it might return, any technical explanations ? If I take it to the service center I would just have blank SAs staring at me
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Old 11th December 2011, 13:07   #2
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Default Re: Car moving faster on applying light brake

There are two possibilities

1. Some particles have entered the rear drum, and are binding the brake. On applying the brakes lightly, the binding is dislodged. Or oil may have coated the brake lining. Initially it just lubricates better, but on harder braking it will engage and brake.

2. The disks of the drums may be a bit off centre (or scored), hence are binding in normal use. With application of brake, they are released marginally.

In either case please get the brakes checked, and aligned. In case the reason is 2), then you have to get the disks/drums machined and the pads/shoes centered. Procrastinating may damage them beyond salvage.
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Old 11th December 2011, 20:37   #3
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Default Re: Car moving faster on applying light brake

Its just a feeling, you get in cars which has improper brakes.

If you are skeptical, do a complete wheel service.

How much kms on the oddo. when did you last serviced the wheels.?
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Old 11th December 2011, 23:18   #4
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Default Re: Car moving faster on applying light brake

The car is 60k on the odo, just recently had a service at Suhrit Hyundai who replaced the brake cylinders and brake discs, I will try and get the detailed jobsheet and post it here. So it should definitely have good brakes
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Old 12th December 2011, 10:22   #5
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Default Re: Car moving faster on applying light brake

Check whether the brake booster is working fine or not. Also check for air in the system - have the system bled as per procedure. Replacement of brake discs at 60k km sounds rather too early. Which cylinders were replaced? Wheel cylinders (front or rear?) or master cylinder?
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Old 12th December 2011, 22:37   #6
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Default Re: Car moving faster on applying light brake

Quote:
Originally Posted by f1head View Post
The car is 60k on the odo, just recently had a service at Suhrit Hyundai who replaced the brake cylinders and brake discs, I will try and get the detailed jobsheet and post it here. So it should definitely have good brakes
Please do post the details. I have only answer then, if it was serviced recently they have not bled the system properly.

@SST
Replacement of disc depends upon how timely the pads were changed during those 60k kms. Under normal conditions disc will last way past 1 lac kms.

PS to OT:
did the disc had formed grove on the surface. i mean was the disc surface uneven.?
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Old 14th December 2011, 00:24   #7
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Default Re: Car moving faster on applying light brake

My Front Discs have been replaced atleast 3 times in less than 60000 kilometers. I have Mercedes Benz E Class. Is it because the car is quiet heavy ?
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Old 14th December 2011, 12:55   #8
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Default Re: Car moving faster on applying light brake

Quote:
Originally Posted by supercars View Post
My Front Discs have been replaced atleast 3 times in less than 60000 kilometers. I have Mercedes Benz E Class. Is it because the car is quiet heavy ?
Is it the disks or the pads?

If it is the pads, I do not think that it has to do with weight, else Accord brakes would also require frequent change.

It seems to be a design criteria for German Cars to change the brake linings much faster. May be they want the brakes to function perfectly every time, so do not want to take chances (just like they do in aircraft!)

If you have changed the brake disks, then it is a reason for concern, as I have normally seen the disks lasting the life time of the car, except where they have been scored by bad lining or warped due to accidental damage.
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Old 14th December 2011, 13:13   #9
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Default Re: Car moving faster on applying light brake

A silly thought, please don't shoot.

Is it possible your throttle cable is stuck somewhere and when u press the brake pedal it also gets pressed/pulled?

Does the engine rpm rise when u brake?
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Old 14th December 2011, 13:30   #10
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Default Re: Car moving faster on applying light brake

I have not got involved in any accident till now. The Discs get uneven and instead of getting them cut, I just replace the discs.

I have replaced the Pads more times than the Discs.
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Old 14th December 2011, 23:16   #11
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Default Re: Car moving faster on applying light brake

Quote:
Originally Posted by supercars View Post
My Front Discs have been replaced atleast 3 times in less than 60000 kilometers. I have Mercedes Benz E Class. Is it because the car is quiet heavy ?
This is very unusual if it is the discs. As for the pads they will replace them only if it is worn beyond the indicated thickness. You will get a warning on your dash about brake wear. It will depend somewhat on your braking style as well. If the wear is uneven have your wheels aligned and balanced first. Check if the car tends to drift under braking. Ask the dealer about the cause since it is not usual.
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Old 14th December 2011, 23:52   #12
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Default Re: Car moving faster on applying light brake

Quote:
Originally Posted by sai_ace View Post
A silly thought, please don't shoot.

Is it possible your throttle cable is stuck somewhere and when u press the brake pedal it also gets pressed/pulled?

Does the engine rpm rise when u brake?
There is no rpm rise, just a clutch like feeling. It happened only once, predictably the SAs at the service center were clueless. I will attach the job sheet as soon as I can get some time at office. Thanks for the suggestions
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Old 15th December 2011, 11:07   #13
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Default Re: Car moving faster on applying light brake

Clutch like feeling can be due to expectation that you want the car to slow down after brakes are applied but it doesnt due to its ineffeeftiveness.
If Does the RPM drop? (That happens when the clutch is applied). Just to rule another silly possibility
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Old 15th December 2011, 11:21   #14
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Default Re: Car moving faster on applying light brake

Have you checked the mileage? Any significant drop?
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Old 15th December 2011, 11:24   #15
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Default Re: Car moving faster on applying light brake

Could this happen when the ECU feels that the engine is going to stall and try to pump in fuel to increase RPM? I have got this feeling in my Santro as well, but this was when I had almost slowed to a crawl and had delayed pressing the clutch to allow for complete engine braking.
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