MOT Workshop 20 – Brake Testing for the MOT

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Here we take a more critical look at MOT brake Testing. First we’ll have a quick jog through the highlights of the test pointing our some of the pitfalls and traps for the unwary, then we’ll ask the question, “could and should the MOT examination of the brakes be improved?”

Note to MOT Testers: The following article appeared in November 2007 and information contained in it may not therefore be current – always check the MOT Manual if in doubt about any aspect of the MOT Test.

As much of the ‘methods of inspection’ and ‘reasons for rejection’ are straightforward, we’ll look at the brake test, where problems could arise.

On the parking brake, whilst the ‘method of inspection’ is fully detailed for handbrake mechanisms, the Tester is given much more latitude for foot-operated systems. The manual says… the ‘Method of Inspection’ detailed will need to be varied for the particular mechanism”. Plenty of latitude there then.

Perhaps the greatest problem, especially on later vehicles, is that some handbrake mechanisms simply can’t be fully inspected. From within the vehicle they are enclosed, normally within the central console, and from beneath the mechanism can be obscured by heat shielding above the catalytic converter.

Perhaps technology has overtaken the Manual. When this section was originally written it was possible to see most of the mechanism on most vehicles – not now. So if there’s a problem with the brake ‘bottoming’ you may well not be able to say precisely the cause.

So selecting a failure when the handbrake ‘bottoms’ can be problematic.

Strictly speaking you should select on the screen: “When the brake is fully applied there is no possibility of further travel of the lever because the lever is at the end of its working travel on the ratchet.” – or one of the other specific ‘reasons for rejection’ offered by the computer – otherwise you could get a disciplinary point on this.

So what do you do if you don’t know the specific reason? – Easy, make sure you make an advisory note!

Service brake inspection

Whilst straightforward, there are some regular pitfalls for Testers here. Make sure you check the footbrake for sideways movement – and you may have to go down on your hands and knees to do it!

Again, be careful when selecting a ‘reason for rejection’ on the computer if you believe the pedal depresses too far before engaging the brake, you must select “When the pedal is fully depressed there is not enough reserve pedal movement”. Although also remember the ‘pass and advise’ rule. How far is ‘too far’? Would it be better to pass and advise?

motw20bt1-2Even if the vehicle has passed the MOT Test, when the brake cylinders are exposed by removing the brake drums you may find that they are ‘weeping’ brake fluid, as here, and need replacement.

Another ‘trap’ for the unwary is to make sure that you check for leaks both with and without the brake applied. This seems rather silly when any leak will be much more likely to be detected when the brake is applied – so why check when it isn’t applied? We don’t know but it’s in the Manual so you must do it!

Although the fluid level has to be checked if there’s a transparent reservoir or an indicator to see if the level is below the minimum, you can’t remove the cap to check if the fluid level isn’t visible. This seems to take the ‘no dismantling’ rule too far and in road safety terms is seriously at odds with being able to remove the petrol cap to check the seal condition…

Flexible brake hoses must be rejected if “excessively chafed damaged or deteriorated”, but be careful, if the outer covering is cracked or chafed that’s not a ‘reason for rejection’ unless it is severe enough to expose the reinforcement – VOSA’s interpretation of ‘excessively’.

Brake performance

motw20bt1-1Before using the roller brake test equipment, make sure that the vehicle isn’t either in the list of vehicles that you should use test using a decelerometer or plate brake; or doesn’t have some sort of ‘quirky’ braking system that may be damaged using the roller brake test equipment.

Right at the beginning of this section there seems to be an anomaly. There are four examples of vehicles which should not be tested on the roller brake test equipment. They are vehicles with:

  • more than one driving axle permanently engaged
  • limited slip differential
  • belt drive transmission
  • brakes for which the servo operates only when the vehicle is moving.

The alternative is to use a decelerometer or ‘plate brake test’ equipment.

However, later in section 3.7 under the heading “Testing transmission handbrakes” it says “When using a decelerometer to test a transmission handbrake…” yet a vehicle fitted with such a handbrake isn’t in the list!

This isn’t the problem it may seem to be. The reasons listed are only ‘examples’. So provided you’ve got a good reason to do so you can opt to test any vehicle’s brakes with the decelerometer.

But still be careful when deciding to use roller brake test equipment. There are, in particular, some older ‘classic’ and vintage cars out there with very strange braking systems indeed – so if in doubt, check. The very last thing you want is to damage the brakes/transmission on an expensive vehicle!

Roller brake test

When it comes to the use of roller brake test equipment, make sure you do carry out all of the various checks required in the manner laid down in the manual. In particular when checking that the braking efforts both increase and reduce at the same time (i.e. ‘imbalance’), wheel ‘lock up’ must be avoided – and before starting this part of the test, don’t forget to run both rollers so the vehicle is properly aligned.

Vehicle weight

Not all vehicles are listed on the brake data charts. In section 3.7 of the Manual it states that “…the service brake percentage efficiency is considered satisfactory providing the wheel lock occurs on more than half of the wheels braked by the service brake” – a strange wording since that inevitably means three wheels for a four wheel vehicle and two for a three wheeler – so why not say so! The Manual also notes that if you do not get appropriate wheel lock as required, then “…carry out a further brake test using a decelerometer…”.

Future developments

In future hydraulic brakes could become a thing of the past. With vehicles’ electrical voltage increasing, in the not too distant future we will probably see fully electrical brake operation.

Of course the problem of ‘no dismantling’ remains, and there’s also the added difficulty of testing the electronic systems, as the brakes will inevitably be computer controlled. But that’s nothing new; current systems like adaptive cruise control and traction control apply the brakes without driver intervention.

So brake testing is already inadequate because we don’t test the electronic systems currently installed in some vehicles and in the future with full computer control, diagnostic checking of braking sytstems will be an essential aspect of MOT Testing of a vehicle’s brakes.

This is what the European DELSY project is all about. It won’t be that long before we will indeed need ‘approved’ MOT diagnostic equipment to Test the brakes!


A Better Brake Test

We all know that passing the MOT is only half the story when it comes to the condition of a vehicle’s brakes.

Seeping wheel cylinders, broken or stretched springs, thin brake linings and so on could all potentially result in brake failure – yet at the time of test, brake performance could be entirely satisfactory.

So could the MOT brake check be done better?

Safety

“No dismantling” is a golden rule of the MOT, but for brakes is that simply unacceptable if we are to have a meaningful ‘safety’ examination?

Just look at the most obvious and simplistic safety check on a hydraulic braking system – is there any brake fluid?

motw20bt1-3You must unscrew the petrol tank cap to inspect the seal…

We all know the answer. If there’s not a transparent fluid reservoir, then you can’t remove the cap to look and check. Yet this is a nonsense when we are required to remove the petrol cap to check the seal. And this really is about safety, no fluid – no brakes!

Few people managing an MOT test and repair garage won’t have come across the classic MOT/service problem on brakes.

“Yes sir, your car did pass the MOT when you were here earlier before booking it in for a service, but after removing the brake drums we’ve found you need replacement wheel cylinders and brake shoes”… The customer isn’t happy: “but it passed the MOT!”

We all know the problem. Whatever the brake performance the wheel cylinders could be ‘weeping’ and about to fail, and the linings could be worn thin. An MOT brake check says little at all about the condition of the brakes.

Quirks and Inconsistencies

Some vehicles have transmission handbrakes. The Manual explains how to test these using roller brake test equipment. On the other hand using the brake rollers could, on some such vehicles, damage the transmission. So what to do? You just have to decide for yourself… but don’t forget that ‘advisory’ note – and if you do damage the vehicle’s systems, then that’s down to you, you will have to cough up!

motw20bt1-4…but you may not unscrew this one to inspect the brake fluid level, even if the container is not transparent, as shown here.

You have been presented with a vehicle with a damaged tyre for Test. Aha! it says something about this in the Manual under ‘Brake Performance’.

After listing examples of vehicles which shouldn’t be Tested with a roller brake tester, it says “These vehicle should be Tested using a…decelerometer…”. Then, the next paragraph says “A roller brake test is also not appropriate for vehicles with damaged, under-inflated or studded tyres”.

So there it is, we must use a decelerometer.

No! If the tyres are damaged or under-inflated that is most likely to be “Grounds for refusal to carry out a Test” because the vehicle is “… not fit to be driven… “, as laid down in the Guide. Yet the Manual could lead you to mistakenly believe a decelerometer test is required – potentially dangerous.

We raised this with VOSA’s John Saker at their Bristol HQ who said “you are the first person that has mentioned this, I feel this is because generally Testers are experienced enough to know when a tyre is or is not suitable to take on the road…”.

Review

It really is arguable that on the grounds of road safety, the methods recommended to MOT Test brakes should be reviewed. It may also be worth sacrificing that ‘no dismantling’ rule for the better protection of motorists against encountering ‘out of control’ vehicles because they have serious latent brake defects, which the current MOT Test cannot possibly detect.

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