What causes L and R rear disc brakes to brake unevenly?

Arthur

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Aug 7, 2003
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22,333
#1
Popped in to the local licensing dept today to get a roadworthy cert for the Hundy i20. It's in great nick, 4y old, 30k on the clock.

I thought it would sail through, but it plugged because the LR and RR braking performance test (on the rollers) was outside the allowable difference of 30%.

Actual braking on the road is perfectly smooth, no judder, no pull to any side with hands off the wheel, etc, etc - all is normal in every normal driving test I know of (yes I know most braking is on the front anyway). No sponginess whatsoever, so I doubt there's air in the hydraulic line.

Discs themselves look good, unscored, normal wear pattern, etc. Drove a little and braked a few times to heat up the disks - the rear ones seems equally warm to me, so seem to be converting kinetic energy into heat at much the same rate.

New (disc) pads all round 8 months ago, in October.

I'm perplexed.

I took it round to the local Supa Quick (who fitted the new pads) and explained what the roadworthy tester said. They're befuddled, as am I. There's nothing to adjust.

What on earth could cause the LR and RR brakes to have different performance? Is there is crossover/valve in the rear hydraulic brakeline that perhaps might be faulty? Could discs or pads be that different? And what do we do to get them 'balanced'?

I have 14 days to get this fixed and get the re-test.

Anyone got any bright ideas what I should be checking? And how do I get it tested before going back to roadworthy dept?

(Or is the rw tester schnaaing me?)
 

bheinche

Senior Member
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Dec 11, 2015
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#2
It could be that the rollers affected the EBD function of the braking system as it's function is to distribute braking force where the system seems fit
 

Arthur

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#3
Good thinking. Never thought of that ... and wonder whether the Hyundai i20 has an EBD system. Will try to read up later.

If that's the root, and the testing rollers are not even, then I'm snookered. :(
 

Sinbad

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Jun 5, 2006
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#4
Check that discs and pads on both sides are within spec.
Check rubber hoses for signs of wear
Check that the calipers slide freely on their locating pins

There's no crossover valve - left and right are on completely separate hydraulic circuits to provide braking in the event of one circuit failing

Was it handbrake test or footbrake test?
 

Arthur

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#6
Check that discs and pads on both sides are within spec.
Check rubber hoses for signs of wear
Check that the calipers slide freely on their locating pins

There's no crossover valve - left and right are on completely separate hydraulic circuits to provide braking in the event of one circuit failing

Was it handbrake test or footbrake test?
Footbrake. Handbrake is exactly matched so passed.

Will get it on a lift tomorrow, pull the wheels, and check with my own eyes. I especially want to see the pads and discs on both sides.

---
[MENTION=391137]bheinche[/MENTION]. Apparently the i20 grocery-getter has an EBD.
 

bheinche

Senior Member
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Dec 11, 2015
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#7
Footbrake. Handbrake is exactly matched so passed.

Will get it on a lift tomorrow, pull the wheels, and check with my own eyes. I especially want to see the pads and discs on both sides.

---
[MENTION=391137]bheinche[/MENTION]. Apparently the i20 grocery-getter has an EBD.
That would be my main suspicion then, but as [MENTION=15693]Sinbad[/MENTION] mentioned check your disks and pads, rubber hoses and if the calipers are up to spec.
 

Arthur

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#8
Will report back tomorrow, just to close the loop.

What really irritates me is that this vehicle is meant to be an appliance. Fiddling about with anything more than a petrol cap is a pain.
 

airborne

Executive Member
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Jul 13, 2007
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7,656
#11
I had the same fail with the handbrake, took it to the mechanic and sorted after an hour or so, cost R200. You could possibly have some air in the one sides brakeline, just needs a bleed, I'm pretty sure a bleed would help.

You don't need to make miracles just get them under 30%, what was the tested difference?
 

rietrot

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Aug 26, 2016
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#16
There's no crossover valve - left and right are on completely separate hydraulic circuits to provide braking in the event of one circuit failing
you sure about that because I would think that's the only thing it could be?

I don't see how it can be the disk or pads, It has to be the hydraulics.
I would start with just bleeding the brakes seeing that they worked on them. Test if you can? Don't know how you can easily test that. Then replace seals for the break cylinder.
 
Last edited:

rietrot

Executive Member
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Aug 26, 2016
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#18
If you don't know about separate circuits you probably shouldn't be giving advice
I haven't seen the i20s underneath yet. Separate circuits or not my advice is valid.
I'm willing to bet it is a hydraulic problem and bleeding the breaks wil likely solve it. Or else if there is dual circuits then one of the break cylinders is working harder than the other ones.
 

RandomGeek

Well-Known Member
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May 14, 2015
Messages
149
#19
Is the tyres on the left and the right the same brand? Still shouldnt be a massive difference even if it is the case
 
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