Brake imbalance...

Hi there

Thanks to baz657 for sorting me out and allocating me to the owners group.

A couple of weeks ago I took my wife's Golf GT TDI (2001) for its test, which it failed as detailed in the Subject line above. The tester was happy for me to observe the test from a safe distance, and during the brake test he said there was an imbalance across the front brakes. The equipment was showing an imbalance of 28%. Asked what the limit was, the tester replied "25%". It seems that the pressures were rising unevenly but were even at maximum pressure.

The test had been going well until this point, and the tester was clearly disappointed at the result. He tried it again but the result was identical to the first try. With a 21 year old car you have to be prepared to be philosophical, so I opted to take the tester's advice and fit new calipers, discs and pads, and return for retest.

While waiting for parts to arrive, I did some online searching, using search terms like "MOT fail brake imbalance" hoping to find confirmation that guide pins or sticking pistons would be thrown up in discussions. As well as helpful practical posts, amongst the results was this website and in very short order I found myself reading a Matters of Testing blog, indicating that the imbalance limit was being raised from 25% to 30% (this was in 2013 as I'm sure you'll know). I was not able to find any indication that it was subsequently reduced once more to the 25% which my tester had indicated to me as the threshold, so I can only assume it was 30% at test and at retest.

I then spent many hours on your website reading the concerns of testers in general and with regard to changes being proposed back in 2013, while I waited for the parts to arrive. I have to confess that I'm still confused as to how exactly an imbalance fail is issued, so I'm asking the question directly: Does the test computer spit out the fail notification directly, as a result of the brake test, or does it rely on an input from the tester? If there is an input from him, would that input trigger a fail if he were to apply a 25% threshold rather than the correct 30%?

At the retest I took a copy of the special notice which was circulated at the time, and had a friendly discussion with the tester about the test procedures. I do mean friendly - we're still on good terms (I need him - my wife and I have 3 vehicles and a trusted tester is essential in my book) but despite a lengthy discussion I didn't come away with any clear idea as to how the imbalance was deemed to be a fail.

I think I must have been 19 years old when I took my first vehicle for an MOT. I'm now 72 and must have presented (a) self prepared vehicle(s) for test in every year during those 53 years. In all of those years I have listened to, and learned from, the comments of many testers, and while I may not always have liked what I heard, it has always been clear how the advice fitted with the regulations. Until this year!

I've read much about testers' dissatisfactions with the present arrangements for testing and about the anomalies which can be thrown up. In particular, I've read much about inappropriately issued fails, though this was mainly before 2013. (I think!) I'd like to think that this fail was the result of just such an anomaly, something which the system produces as an inevitability, perhaps due to poor software implementation or incomplete tester training. But I simply don't know, and I can't know, and that's why I'm looking for your help here. My tester is in every way thorough, ready to take time to answer questions on side issues, competent in using the equipment, and makes me feel confident that my vehicle is being scrutinised properly, and never merely nodded through. So I need to know what to discuss with him in another couple of weeks, when another vehicle is due for test.

Just to complete the picture, I'm satisfied that air in one circuit was in large measure the cause of the imbalance. The rear calipers had been replaced a year ago, and it's pretty clear that a pocket of air was lurking in there.

My apologies for the length of this post and I hope it's all clear enough. Thanks for reading!


John