MOT fail: new spheres fitted. Retest fail rear brakes delay.

[Thregwort]

This year the Xantia failed (first time ever) due to no movement in nsf suspension. This was the only fail item on the MOT printout.
After much help from this forum, I successfully fitted two new front spheres, filled up with LHM, checked for leaks after a few 'Citrobics' push-ups, and made my way back to the MOT test centre. On the way, I did a few 'emergency stops' and the car seemed to stop as normal.

The retest at the MOT station was not a success - when the rear brakes were tested on the 'rolling road' the mechanic said that there was no immediate response - then he 'floored' the pedal and everything was fine. He also said the under-bonnet 'ticking' was probably a symptom of something. Not a lot of ticking but obviously not completely 'settled' as formerly.

I've read the stickies and numerous related posts on this forum and it seems like the remedy could be anything from a quick-n-easy bleed the back brakes' nipples to a major strip-down acc sphere-change, hydraflush, poss rear spheres change, etc., etc.

As the car was rear-brakes-OK to pass MOT before the front spheres replacement, and not OK after, might any of you experienced contributors be kind enough to offer an 'easiest first' probable diagnosis, based on the details I have given?

Thanks in advance

[CitroJim]

Delays in the back brakes is a common enough symptom of air in the system.

I would recomemd a good bleeding session, remembering that you need to bleed at least half a pint from each rear nipple because of the length of pipes.

I'm worried that the MOT tester did not realise what the ticking indicated. If the tick rate from the pressure regulator is less than 30s than this indicates a worn accumulator sphere and this must be replaced.

Also, if you have no record of it being done, replace the mis-named anti-sink sphere. It is in fact a rear brake accumulator.

Good accumulators are critical to good brake performance.

If the problem is not hydraulic in nature then the calipers might need a bit of a clean up to ensure the pads are both wearing true and are free to move.

Hopefully a good bleed will secure you an MOT pass..

[Chris570]

I always thought that on a retest they could only look at the bits that failed...

given that you've had the system open its most likely air that's got in

[citroenxm]

Defenatly! They should NOT have tested the rear brakes because they passed last time...!!

What you can do firstly, is lower the car to the ground completely, then while the engine is still running, leave it running and undo the Pressure Regulator screw again... leave the engine running for about 5 mins, this then still keeps the fluid running through the whole system, but doesn't build pressure, this then removes air from the main system... if there was no air in the rear before then its most likely around the brake douser valve air has got trapped and leaving it run with the regulator "Open" should clear it...

Ove done, "Close" the regulator, put the car on high, remove a rear wheel.. LEAVE the engine running and Jam the brake pedal on with a bar or something..

Then open a blead screw on a rear caliper but be warned FIT A BLEAD PIPE as it may shoot out at pressure!!

This then will make sure you have no air in the rear, close the blead nipple, but keep an eye on the LHM tank, dont let it get too low!!!

Paul

[Thregwort]

Wow, I'm getting a bit hazy about where all these items that you regulars are referring to are located.

Is there an 'idiot's diagram' somewhere that shows the basic six-sphere layout, with location of the 'screws' and bleed nipples etc.? I'm assuming the bleed nipples for the brakes are on the brake callipers not on the acc spheres - or both need doing?

Lil' help guys?

[citroenxm]

Yes yes.... Brake callipers as normal, remove a rear wheel, you cannot miss the nipple!!

Regulator is on the front of the engine, attached to the front of the gearbox actually... theres a 12mm nut VERY IMPORTANT just slacken it, DO NOT REMOVE....


Its the only 12mm nut on there..

Paul

[citronut]

Chris570 wrote,
"I always thought that on a retest they could only look at the bits that failed..."

only if you get it back in the 10 working day's not including weekends and bank holl's,

after that period they have to do a parshall re/test, wich is almost a cursory look over the hole car again

regards malcolm

[Thregwort]

That sounds straightforward - thanks v much for the super-fast service here.

One last question - how will I know for certain (how can I test) that the bleeding will have done the job when I've finished the work.
As I mentioned above, my previous 'road test' (emergency stops) all indicated everything was good, it was only on the separate-front-and-back MOT tests that the rears were found non-functional.

I don't want to take the car back only to find that it will fail again!

[citronut]

take it back and ask them to stick it on the brake roller before they log it back on the VOSA's system,

allthough 99% certain this will sort it out, also you need the car fully up on its highest setting to get a full flow of citro blood out of the rear callipers,

regards malcolm

[Thregwort]

Thanks, much appreciate your help.

[MikeT]

The way the hydraulic system works means that the brakes are "balanced" according to weight, with the fronts doing most of the work with just a driver and empty boot. If the rear suspension is heavily loaded, the balance is shifted to the rears so they work harder. You therefore might want to add some ballast to the boot when you re-mot it.

[citroenxm]

A lot of testers - and people too - do NOT know that Hydraulic Citroen brakes are NOT split diaganally like a conventional car. IE: Front right Rear Left, and so fourth. ... the brakes are favoured for FRONT only..

Giving the system a bleed will reassure you that theres no air in the system, when you go back for re-test put a couple of breeze blocks or a couple of flag stones in the boot... DONT say anything... this then will raise the brake pressure in the rear brakes and should make them work a bit better..

Rear brakes work according to how much system pressure is in the rear.. the more weight in the boot, the more they work..

Paul

[Thregwort]

So, the more load in back, the more braking to the rear pair - clever.
A brace of tips from the wise - thanks guys - I'll certainly follow your recommendations - I've got a couple bags of cement ideal for the job, I'd have thought...concealed, of course...now, if I could just remember where I put the rear tonneau cover (my Xantia is an estate!!).

I'll get the job done this weekend and post back. Thanks again for your interest and helpful thoughts.

[myglaren]

No need to hide anything in the back. Leave it visible.

I just had my C5 tested and no mention was made of all the crap I have in the back of mine, tools & junk weighing at least as much as a couple of bags of cement.
Wasn't there for the benefit of the test, it lives there most of the time

I've found with the GS, BX, Xantia and the C5 that periodically loading them up to the max - and more, improves the suspension and brakes quite radically.

[xantia_v6]

So, the more load in back, the more braking to the rear pair - clever.

Although that is true, it makes no difference to the MOT test, as this takes the axle weight into account.
I have tested this in practice, I had a Xantia which had poor braking efficiency on the rear (only just failing), and I had a receipt for "high performance" pads being fitted, so they probably didn't have much bite when cold. Anyway I went home and put 150kg of steel and concrete in the back of the car and took it straight back for a re-test, and it failed again with exactly the same reading on the brake machine.
(In the end the tester ran the machine with the brakes on for a couple of minutes and magically the efficiency improved...)