MOT fail but puzzling

[KennyW]

Hi guys,

Took the old girl for her yearly official check up and the same problem from last year has caused the fail.

Rear nearside brake efficiency is below 50% and that is it.

Last year I fitted new seals on the pistons and new brake pipes to R/N/S and R/O/S.

Could I have not completely purged the system of air hence the problem or is it the Ant sink sphere.

I need to replace the 3 rear sphere anyway.

Any thoughts or suggestions please,

Kenny

P.S. I know the MOT man very well and he informed me the are new brake regulations coming in on the 23rd June re efficiencies so be aware.

[Mandrake]

Does yours have the rear callipers out of line due to corrosion or have you dealt with that before ? If you brake hard does the rear suspension rise up ?

Either way I wouldn't worry too much, the rear brakes don't get much work if you drive smoothly and don't carry heavy loads, and on the salt laden UK winter roads it wouldn't take much corrosion around the pad area for the pads to seize up. My pads were well and truly rusted to the callipers so they couldn't move when I got my car!!

I'd just do the usual:

Remove the pads and make sure they aren't rusted to the calliper, scrape off any rust so they don't bind in the slot in the calliper then refit the pads. Then use a pair of these:



To squeeze each pad right back, with these adjustable pliers you can easily squeeze the piston back with the pad still fitted by squeezing between the tab at the top of the pad and the calliper housing... then press the brake pedal to push them out again.

I squeeze each piston in 3-4 times to work them all fully through their range of motion and free them up. Finally bleed the rear brakes.

Throw a couple of bags of cement into the boot and go back for a retest and I'm sure she'll pass with flying colours.

Some people might consider adding weight in the boot cheating but it really isn't. The regulations were not written with weight adaptive rear brakes in mind, and although perfectly working they should pass with no extra weight when you factor in corroded callipers and sticky pistons it can be marginal with the boot empty and even a full tank of fuel can add enough weight to pass where it might have marginally failed with a near empty tank.

Don't leave it to chance, make sure the boot is well loaded for the re-test, the MOT man doesn't need to know.

PS if you find a lot of "air" when you bleed the rear brakes and the anti-sink sphere is due I would replace it as punctured anti-sink spheres can stream a slow but steady amount of nitrogen bubbles into the rear suspension which tends to collect in the rear brake lines.

Thanks for the heads up on the coming change in regulations... knowing that I think I'll preemptively give my rear brakes a once over as above and throw some weight in the boot before I take mine in for MOT at the end of July...

[KennyW]

Thanks Simon,

I just remembered I didn't change the seals on the R/N caliper, the other was fully refurbished last year.Corrosion build up dealt with last year, hence the one refurb caliper.

Will get on with it,hopefully, on Sunday.

See you are getting there with the misfire keep it up, I think you've finally cracked it.

Kenny

[Mandrake]

Thanks Simon,

I just remembered I didn't change the seals on the R/N caliper, the other was fully refurbished last year.Corrosion build up dealt with last year, hence the one refurb caliper.

Personally I wouldn't have thought that the calliper seals would have any bearing on how well the brakes would work. I would only replace them if there was a leak from the piston, and I've never had to replace any on any car I've ever owned. More likely the act of taking the piston out to replace the seal is what helps free it up, just the same as when I work the pistons back and forth through their range of motion.

Will get on with it,hopefully, on Sunday.

So you have crap weather predicted for today and tomorrow clearing up on Sunday too ? I think my fuel delivery test will have to wait for Sunday too...

See you are getting there with the misfire keep it up, I think you've finally cracked it.

Kenny

Thanks,but given the number of false alarms so far where I thought I'd cracked it but I hadn't I'll be waiting for the fat lady to sing, go on vacation, and come back and sing some more before declaring victory.... this car will not give in easily, it will be kicking and screaming to the very last I suspect...

In terms of amount of time spent troubleshooting and number of not faulty parts replaced the most difficult car I've ever owned, by far. The only thing that came close was an '84 daihatsu charade turbo whose carburettor was not tuned and working properly, I never did get to the bottom of it in the time I owned the car, it was a complete plumbers nightmare around the intake system with dozens of small vacuum pipes mixed up and going god knows where with no documentation or routing diagrams available, and nobody local would tackle the problem either, not even carburettor specialists!

[Peter.N.]

If you can lift the caliper boot away from the cylinder a little and squirt some LHM into it this should free it up with use.

Peter

[KennyW]

Thanks for the reply Peter,

Hopefully I can get the old girl on the road and sorted plus I'm looking to finish of the Focus clutch. What a pain to get gearbox back on, the access is very tight, that's what is hold me on that

Simon, It's amazing how we are only 30 miles away but the weather can be so different between East and West.

Kenny

[Mandrake]

It actually wasn't too bad yesterday, I was out measuring the fuel pump voltage and checking the fusebox. Today is wet and miserable though...

[KennyW]

Simon,

It was fine all day until the evening when the showers appeared (Saturday), today got the rear brakes bled and changed a piston.

But, the focus is another story

Will speak to my MOT man tomorrow and hopefully she'll sail through.

Kenny

[KennyW]

Good Morning All,

Well she passed and the efficiency improved but not has much as i thought.

Kenny

[Mandrake]

Did you use the bags of cement ? Or did it pass without any extra weight...

[Northern_Mike]

Did you use the bags of cement ? Or did it pass without any extra weight...

Am I the only one who doesn't have a couple of bags of cement lying around for MOT purposes? The mind boggles.

Sent from my GT-I9300 using Tapatalk 2

[Northern_Mike]

Does yours have the rear callipers out of line due to corrosion or have you dealt with that before ? If you brake hard does the rear suspension rise up ?

Either way I wouldn't worry too much, the rear brakes don't get much work if you drive smoothly and don't carry heavy loads, and on the salt laden UK winter roads it wouldn't take much corrosion around the pad area for the pads to seize up. My pads were well and truly rusted to the callipers so they couldn't move when I got my car!!

I'd just do the usual:

Remove the pads and make sure they aren't rusted to the calliper, scrape off any rust so they don't bind in the slot in the calliper then refit the pads. Then use a pair of these:



To squeeze each pad right back, with these adjustable pliers you can easily squeeze the piston back with the pad still fitted by squeezing between the tab at the top of the pad and the calliper housing... then press the brake pedal to push them out again.

I squeeze each piston in 3-4 times to work them all fully through their range of motion and free them up. Finally bleed the rear brakes.

Throw a couple of bags of cement into the boot and go back for a retest and I'm sure she'll pass with flying colours.

Some people might consider adding weight in the boot cheating but it really isn't. The regulations were not written with weight adaptive rear brakes in mind, and although perfectly working they should pass with no extra weight when you factor in corroded callipers and sticky pistons it can be marginal with the boot empty and even a full tank of fuel can add enough weight to pass where it might have marginally failed with a near empty tank.

Don't leave it to chance, make sure the boot is well loaded for the re-test, the MOT man doesn't need to know.

I'm sorry. I have deleted the contents of this post, as it was Sh!t3

[Northern_Mike]

I've always loaded up the rear at MOT time because otherwise the brakes are only marginal. The rears only really kick in under load, unfortunately the tester doesnt seem to appreciate this. Obviously, with a load they work perfectly and i get a ticket! Cant see how it can be bad advice, if you get a pass on the rears they must be working.

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I've never loaded any of the 5 Xantias I've taken for MOT. They all passed because the rear brakes were clean and working properly.

If they're corroded and have sticky pistons, and only scrape through, then they're not working properly or in decent condition.

The MOT brake test is a bit of a sham anyway. One of my MX5s passed with brakes in such poor condition that I put new calipers on the front before I sold it afterwards. The DRK passed in April this year with brakes that grabbed so badly they'll literally rip the steering wheel out your hand if you're not ready for it.

People seem to happy to accept the MOT as proof that all is good with a car, when it's really just a minimum standard that a vehicle has to meet to be allowed on the road. For some parts, including brakes in my opinion, it's pretty low.

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[Mandrake]

I'm sorry. This is really bad advice. You're effectively telling someone how to cheat the brake test when their brakes aren't working properly - I count "corroded calipers and sticky pistons" as a fault that should be fixed properly , not bodged.

If you want to be a pillock and not repair your brakes properly, that's up to you, but please remember there are others on the road.

You really need to step back and calm down Mike, there's no need to accuse me of being a pillock, especially when you're misrepresenting what I said.

Nowhere did I suggest anyone "just" throw extra weight into the boot to pass the test. I clearly listed several steps that should be taken first to rectify the performance of the rear brakes to get them working as well as possible. Steps that in the vast majority of cases (assuming discs and pad surfaces are in decent condition) will resolve any efficiency issues.

However I stand by my statement that the MOT requirements are not written with consideration of weight adaptive brakes which puts lightly loaded Citroen's at an unfair disadvantage. 98% of the cars on the road being tested are capable of applying full brake pressure on the rear brakes regardless of load, and rely entirely on ABS to control any potential wheel lockup.

A Xantia with no load in the boot only has about 55 bars of rear suspension pressure (yes I've measured it) available for the rear brakes compared to 170 bars for the front brakes. That's a huge difference not even considering the difference in piston/pad/disc sizes between front and rear.

80% of the braking effort on an unladden Xantia is provided by the front brakes, the rear brakes are only really there to apply just enough bite to keep the rear end from wandering and keep the car directionally stable. Any more than that and the wheels will just lock up and trigger the ABS anyway. In a heavily loaded car its a different story, and that's exactly when the rear brakes will be allowed to work at a higher pressure. Fully ladden the rear brakes receive well over twice the pressure they do when unladden.

Far from "cheating", putting some load in the back of a Citroen simply allows it to play on a more level playing field with other cars during the test. You're simply allowing the brakes to work to their full potential. That the rear brakes might not be as efficient as other cars lightly ladden is irrelevant because in a real world braking scenario when the cars inertia transfers most of the weight to the front wheels (unlike a stationary test on a brake machine that doesn't cause weight transfer) that extra "efficiency" only leads to the rear wheels locking up (sans ABS) which is why Citroen limit the rear brake pressure with light loads in the first place!

If you want "cheating", how many people take their diesels for an "italian tuneup" before their MOT to make sure it passes the emissions test ? How many people wipe off any LDS wetness on their C5 suspension struts before their MOT to avoid comments by the MOT man ? I suppose they're pillocks too ?

[mendedit]

Acquiring a new MOT using bags of cement!
I'm afraid I'd want something more Concrete than that.