MOT brake failure Q

[Mr_Bo11ox]

Alright shiters

 

My 'Crab failed the MOT today, on 'Low service brake efficiency 41%'. I tried to press the fella about what exactly that meant but i think he felt like i was trying to make him commit to a fault diagnosis there and then, he spluttered a bit then clammed up completely, not much help but thats what you get for taking your snotter to one of those 'F1 autocentres' I suppose. I have used this place before and the guy is amiable anough but seems to be frightened of customers and is a dreadful communicator! The test was only £29 - peanuts, monkeys and all that. Anyway I now have to sort it out. The failure seems to be simply that the brakes are not good enough!!! Its not a case of an imbalance or one wheel not working properly or whatever, the brakes are apparently just not powerful enough all round.

 

I'm at a bit of a loss TBH as i renewed the entire braking system last year, including even the servo. The brakes do require a good shove alright, but i just assumed that was normal for an old car. They can lock the wheels (if you push really hard) and dont drag you one way or the other during an emergency stop. The servo is working, if you pump the pedal a few times with the engine off, then start the engine up, the pedal sinks a bit which i think tells me its doing the biz. The M/cyl is in good order, its not one for an 1800 but its the right bore and is the same one I passed the MOT with last year. Short of bleeding them through (i do think there might be a tiny bit of air in there) and maybe replacing the ancient front pads i'm a bit stumped, and i'm tempted to go to another MOT shed and ask them to evaluate the brakes for a bit more insight. Any tips?

[The Reverend Bluejeans]

Sounds like it's under servoed......? Is that 41% all round - any imbalance at all? Does this one have the single piston sliding calipers? Could be that they want taking off, the sliders coppergreasing, a bit of WD40 behind the piston seals and some pedal pumping. It's not inconceivable that the front calipers are seizing.

[scaryoldcortina]

Low service brake effiiciency... hmm. I have a feeling that someone didn't use the computer properly.

 

Here's how it all works.

Each wheel has its brake force measured (in kilogrammes) then all four readings are added together and divided by the "brake test weight" of the car - this gives you your percentage efficiency, 50% is the pass mark. If at least 3 wheels lock on the rollers, it passes regardless.

 

Problem is, how to determine the brake test weight if you didn't get 3 locked up wheels... for class VII it's easy, MGW/MAM from the vin plate. For class IV (cars) though you use a special figure provided by VOSA (it works out at the dry weight plus 130kg iirc) which for cars made since 1980 the computer puts in automatically. Older stuff will need the weight supplying by the keeper, or maybe off the old brake weight charts (if the test station still has them), or decelerometer testing to determine efficiency instead.

 

My guess is that the tester used the MGW from the vin plate to calculate the efficiency... If that is all it failed on, I'd pop back and have a word. If you would like the references for the relevent bits of the tester's manual, just ask...

[Timewaster]

I wonder if the pads and shoes are a bit glazed, If you are stripping the brakes down it might be worth roughing them up with some sandpaper.

It could be an accumulation of little problems.

[wrecker]

did they give you the readings?

[simmo]

Get the readings and if the rears are low stick some weight in the boot so the load, sensor allows more effort, the fronts usually lock out

[Mr_Bo11ox]

did they give you the readings?

 

No they didnt, I think I should probably have asked for them, but I felt sorry for the fella behind the counter. He was fluffing his words and stammering and I felt like a fat, armed US immigration official harassing some poor asian student in an airport about why they looked a bit different on their passport photo. Eventually I just wanted to get out the shop.

[Cheggers]

I'd take it for a retest at another testing place - If the brakes feel good to you, they probably are

[M'coli]

Do you know a tame garage with a brake roller that will allow you to independently test this, even for a small fee?

[Mr_Bo11ox]

Low service brake effiiciency... hmm. I have a feeling that someone didn't use the computer properly.

 

Here's how it all works.

Each wheel has its brake force measured (in kilogrammes) then all four readings are added together and divided by the "brake test weight" of the car - this gives you your percentage efficiency, 50% is the pass mark. If at least 3 wheels lock on the rollers, it passes regardless.

 

Problem is, how to determine the brake test weight if you didn't get 3 locked up wheels... for class VII it's easy, MGW/MAM from the vin plate. For class IV (cars) though you use a special figure provided by VOSA (it works out at the dry weight plus 130kg iirc) which for cars made since 1980 the computer puts in automatically. Older stuff will need the weight supplying by the keeper, or maybe off the old brake weight charts (if the test station still has them), or decelerometer testing to determine efficiency instead.

 

My guess is that the tester used the MGW from the vin plate to calculate the efficiency... If that is all it failed on, I'd pop back and have a word. If you would like the references for the relevent bits of the tester's manual, just ask...

 

Hey Cheers SOC, thats great, i must say i was quietly hoping you might chip in. I did wonder how they arrived at a % efficiency number, I think i will go back and ask them to show me how they worked it out.

[Angrydicky]

The brakes on my crab are absolutely excellent, in fact they're better than the modern Sprinter van I drive every day. They certainly don't need a 'good shove' so maybe that's what your man is talking about?

 

Could be the servo possibly. Mind you the brakes on my Cambridge are pretty terrible but they went straight through the MOT. Could you get a second opinion?

[Mr_Bo11ox]

The servo is actually an MGB one, i dont know if it is a different ratio to a pukka crab one TBH so there could be something in that. It does give the impression that theres not as much servo assistance as perhaps there should be. But, there is defo some and i'm not sure if the level of servo assistance would actually affect the efficiency of the brakes? I think it would just mean you have to press a bit harder to get the same result at the wheels, but the max retardation available would be the same. There may be an unexplained level of retardation in my logic like so bear that in mind.

[scaryoldcortina]

Hey Cheers SOC, thats great, i must say i was quietly hoping you might chip in. I did wonder how they arrived at a % efficiency number, I think i will go back and ask them to show me how they worked it out.

 

No problem! The tester should still have the vt40 testing sheet (blue one) with the brake roller readings written on it. From experience, I'd expect front effort to be around 300-350kg and rears to be close to 200 on a car of that size and age.

[AXrescuer]

Sounds like it's under servoed......? Is that 41% all round - any imbalance at all? Does this one have the single piston sliding calipers? Could be that they want taking off, the sliders coppergreasing, a bit of WD40 behind the piston seals and some pedal pumping. It's not inconceivable that the front calipers are seizing.

 

How would that affect the service brake efficiency?

[oldford]

There is either a fault with the brakes or the braking weight is wrong. If the test was conducted at a station with an Automatic Testing Lane, the car would be weighed and the calculation based on that. If the weight is unknown and it does not meet the locking wheel criteria a decelerometer test should be done.

 

Braking weights appear automatically when the test is logged on, if not, a lot of testing stations will have a wall chart, but I'm not too sure how far these go back. It may be worth asking other owners if they know what the brake testing weight should be.

 

It's all here:

 

http://www.transportoffice.gov.uk/crt/d ... guides.htm

 

Look in manual 3

 

As stated previously the efforts recorded for each wheel will be recorded on the VT40 check list, I would like to see them printed on the paperwork.

 

Is it a dual or single hyydraulic system? If single, the parking brake efficiency needs to be higher, 25% as opposed to 16%. A pass on the parking brake would indicate that the rear shoes & drums would appear to be good.

[wrecker]

at the testing station the computer could have the weight on it ,or there could be a wall chart with the weight on .you add the weight on the correct screen and add the brake figures of each wheel and the computer does the rest and gives you the %.most cars can be 250 -400 on the front and 50 -300 on the back.if you say the brakes dont feel good ,then perhaps there is a problem?

[The Reverend Bluejeans]

Sounds like it's under servoed......? Is that 41% all round - any imbalance at all? Does this one have the single piston sliding calipers? Could be that they want taking off, the sliders coppergreasing, a bit of WD40 behind the piston seals and some pedal pumping. It's not inconceivable that the front calipers are seizing.

 

How would that affect the service brake efficiency?

 

Pretty obvious I would have thought.

 

a) Under servoed, meaning the little MOT guy isn't able to give the pedal full beans. Servos don't increase braking power/ efficiency as such, but they make it easier to achieve full braking power for a given pedal effort. That's why the old Jap stuff with over servoed brakes were so easy to stand on their nose.

 

 

Seizing calipers - meaning that no matter how hard you lean on the pedal, the pistons aren't doing their thing.

 

All clear?

[Mr_Bo11ox]

Well, i went back to the MOT shop and asked how they came up with these brake efficiency numbers. seemingly it was 100% automatic, they weighed the car, did the test and the machine said 'no'. They could not give me individual readings for each wheel.

 

I found the servo was on upside down (i installed it ) which could lead to bleeding problems, so I have corrected that, fitted some new EBC brake pads and bled the lot through again. Now the brakes seem noticeably more powerful and much less hoofing is needed to lock the wheels so hopefully i should get the elusive green bit of paper on Monday!

[Gompo]

Good luck, I presume you're going back to the same place again?

 

Will be interesting to see what happens.