1990's Astra 1.4 Brake imbalance - MOT fail.

Detect it by taking it somewhere less picky...

Front brake imbalance is done on the rollers, if it's unbalanced it's faulty. On Astras new slider pins often cures it, but roll the dust sleeves back on the weak caliper & check for corrosion 1st, if it's not been driven hard then you can get sticky calipers at the end of their travel.

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Not the same thing, I know, but last year my Focus failed on rear brake imbalance. I got the fail notice together with an offer to have a look, and see how expensive the repairs would be! I had fully overhauled the brakes less than 3K miles previously.

I booked a re-test for the following day, took it back without doing anything, and it passed. They declined my request to see the actual numbers.

They were a local garage, 10 minutes walk away, and had done all my MOT's for years. Needless to say, I won't go there or recommend them again.

My new(er) Focus needed a test last week. Through someone who worked in the trade, I found a testing station 20 minutes drive away that only does tests.

I got chatting to the tester; he had worked at the place that had failed the other car, but eventually left. One of the reasons was that he was under constant pressure to fail more cars...

The second bay at the test centre was testing a MB A class. The guy testing that kept driving it on and off the brake rollers, and re-doing the test. "My" tester explained that the rollers weighed the car, and if you didn't drive on them carefully, the weight, and hence the efficiency recorded, would be wrong. Less scrupulous places could easily have just taken the first figure as a fail.

Perhaps you might be able to find a testing only type of place for the future?

As far as I can see, only by paying in one way or another.

Chris

if it is serious enough to fail then you should be able to feel it, ask which brake has the better reading then have a real look at the other side. My money is on a rusty/stuck piston. Pull the caliper off but leave the hose connected, inject a little brake fluid under the piston diust boot, clamp the flexible, undo the bleed nipple and using a suitable clamp press the piston back in expelling fluid. (Ideally it should be possible to press the piston in by hand), having done that do up the bleed nipple, release the hose clamp and gently press the brake pedal a few times to push the piston out again, repeat a few times till it goes in nice and easily (keep an eye on brake fluid level). If it won't free up then strip the caliper and rebuild or replace it. calipers are actually quite cheap these days.

Did you take it to either Kwik Fit, National Tyres or the Vauxhall Main Dealership? Kwik Fit always lie and con people using brakes as an excuse, they used to try it on with shock absorbers. National Tyres are getting just as bad and the main dealer will con you senseless if you allow it.

I suggest you take it to another garage. This problem started a few years ago when companies just wanted to make as much money as possible in the shortest time. In the case of Kwik Fit and Vauxhall dealerships, the managers forced staff to meet a sales target. If they didn't sell, by any means, it would result in disciplinary action. Similar to the way the AA/RAC force people to sell batteries to everyone - as featured on BBC Watchdog. I would appeal against the decision and see how far they go before backing out. Your local council operates MOT test centres in a lot of areas, you can book a test there. They have no interest in claiming the vehicle has failed the MOT in order to carry out profitable repairs that are never required. Also start complaining to your local Trading Standards office and VOSA. Let's eliminate the rogue garages and dealerships.

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It wasn't the AA, just the RAC.

The only thing AA patrols are encouraged to sell is membership.

Chris

It's just the idiot morphing again.

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Thanks for the heads up; hadn't spotted him this time.

Chris

Thanks for all the advice, a follow up to my previous posting,

Yes it was an independent garage, they do very little other vehicle work besides welding so nothing to gain from failing it on the brakes.

I road tested it again and found a panic stop from 40mph caused the drivers side front wheel to lock up right at the end of the stop if I didn't modulate the pedal pressure. From speeds less than this there was no locking. There was certainly no evidence whatsoever of any pulling.

So I removed the calipers from their mounts and exercised the hydraulics against a G clamp, the pistons moved ok and I checked that they returned with thumb pressure I couldn't find any seal kits locally so didn't bother disturbing the dust covers and hydraulic seals any more than looking under the dust seal - there was no corrosion.

I removed the sliders and flushed them through to remove all traces of grease, no wear was evident, no damage to the sliders either so I relubed with copious amounts of CV joint grease, the sliders seemed a bit more free moving than before.

I checked all areas of the caliper that the pads were bearing on and removed a small area of corrosion maybe the size of a drawing pin head and maybe 1/16" thick. Ran a smooth file over the metal bits of the end of the pads to remove any burrs. A few blobs of copper grease on the sliding areas and back out on the road to retest.

It was still locking at the end of a panic stop but the road was now slightly damp so I'm not sure it was representative.

In desperation we took it back for a retest (free) and it passed.

The previous imbalance was 30%, the new imbalance is 17% (which I presume is still high?) In hindsight I might have tried using a rotary hone on the discs and rubbing the pads on some glass paper.

17% is down in the zone from the two tyres being worn differently & at different pressures.

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FYI, the maximum permitted imbalance on the steered wheels is 25%.

Where some testing stations are not being totally honest is how they interpret MOT 3.7A 2c (and d):

"the braking efforts at the roadwheels do not increase (decrease)at about the same rate..."

It's the "about" that allows too much unscrupulous tester discretion.

Chris

sometimes a fresh set of pads can cure imbalance, I believe this is probably where one side has become a bit cooked from minor seizure that has gone unnoticed for a while.

Surprised that could make such a huge difference but I can probably eliminate those reasons too.

Tyre wear is pretty even (On the failed test there was a tyre with slightly excessive inside edge wear which was an advisory so we swapped it with the near new spare)

I checked all the tyre pressures just before the retest. They were all a few pounds high 36vs30 front and 30vs28 back iirc having not been reset after the car was used for towing a few weeks back.

If one sides new & one side is worn or a different make you'll get imbalance.