Car Pulls To The Right When Braking

[silencer51]

Hi all,

Its been a long time since I inquired about a problem with my Megane...

Quick overview of the problem:

Under heavy braking (with ABS kicking in), the car sometimes pulls to the right, noticeably so.

This cannot be replicated every time, the higher the speed and stronger the braking, the more evident the pulling...

The front suspension system has been checked, a wheel alignment has been performed, the brake fluid has been replaced recently, new front & rear disks
have been installed, tyre pressures are ok and technically it all checks out ok.

The last thing I replaced were the rear brake discs. For some reason the ABS sensor ring was missing from one of the (original Renault) discs so the mechanic used the sensor ring from the old disc.

Everything seems to work fine, brakes feel and perform fine, but in the last few days I've had to brake suddenly and with great force a few times and thats when I noticed the pulling.

I went to a professional brake specialist who measured my brakes.

Front brakes check out fine, there is around 4% difference in braking pressure and the graphs are spot on.

On the rear, the difference between left and right is 12% which is rather high (close to the limit). The graphs show that under heavy braking the rear left brake brakes slightly harder than the right one.

Now... Since the rear left brake is 'stronger' than the right one, shouldn't the car pull towards the left? Somebody correct me if I'm wrong.

Also, any advice on what I should do are welcome. Webcode, what is the official Renault spec for left-right brake pressure differences?

Could the new discs and brake pads have caused a problem? Or perhaps the re-used sensor ring? I would think though that if the sensor ring was defective the car's ABS/ESP unit would detect the discrepancy and log a fault...

P.S. The mechanic also tested my handbrake and it needs adjusting, there is a 35% difference between left and right brake pressure when the handbrake is on.

[AlexB]

Hi silencer51,
On which side is the reused ABS ring?
BTW, you are net supposed to use the new brakes hard for the first 500 miles. There are also exist bedding-in procedures.

[silencer51]

The ABS ring that was reused was installed on the rear right wheel.

From what I can gather from Renault's documentation, the ABS computer monitors the rotation of each wheel independently. Now the ABS ring
has 47 'magnetic indentations' around its periphery and these, together with a Hall-effect sensor are used to measure the speed of each wheel.

If the ABS ring was damaged (it was very gently removed from the old brake disc right in front of me) then it would log a fault since for each rotation of the other wheels the 'problematic' sensor would report either greater or lower speed and thus trigger a fault with the ECU.

At least in theory... right?

[AlexB]

The theory looks right...

What about rear callipers? It is not unheard of that they seize constantly or intermittently. Were they bled, BTW? Does the handbrake work and is it equally strong on both sides?

[silencer51]

The system was, ofcourse, bled when I replaced the brake fluid...

My car failed the handbrake test, there was a 35% difference between left and right sides (stronger on the left side if I remember correctly).

I need to get that adjusted...

[AlexB]

So, both the handbrake and the footbrake are stronger on the left side. It looks like a problem with the right calliper.