Citroen C8 Brake Pedal Excessive Travel

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JamesQB
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Citroen C8 Brake Pedal Excessive Travel

Post by JamesQB »

Hi all, looking for some advice please. Citroen C8 2.2 HDi 2005.

Fault: Excessive brake pedal travel. Brakes don't start to have an effect until the pedal has gone down a long way and no real resistance felt until pedal is about halfway down or more. Applying brakes hard means pedal isn't that far from full extent of its travel.

Attempted: I have tried bleeding using two-man method with engine running. No air came out despite lengthy bleeding and using a large quantity of brake fluid. Also carried out secondary circuit bleed using Lexia. Again no air and no improvement at all. Haven't tried using my little pressure bleeding kit (1 Bar max) as last time I used it on my old Passat, the reservoir cracked. Although I know this is due to aged plastic, I don't want the same hassle with the C8 if can be avoided.

Additionally: Before trying to solve the brake pedal travel, I thought it'd be best to change brake pads on all four corners as all worn, replace badly worn rear discs, and clean up each calliper and ensure free movement (one front pad was jammed in the calliper due to rust buildup on slide), then do a full bleed.

However, this has created another issue which seems common enough for the C8 that there are quite a lot of posts on various forums but never any return to say what the solution was: the brake pedal now sinks to the floor if pressure on the pedal is maintained. I tested on a steep hill and the brakes hold the wheels even when the pedal is touching the floor.

I had both these problems on the VW Passat last year - long pedal travel with brakes not gripping until pedal was quite far down, and pedal sinking to the floor with sustained pressure. Despite many bleeding attempts in the past including with the pressure bleed kit, this solved it: two-man bleeding but using one-way valve built into Draper fluid collection bottle, engine running which I'd never tried before, and passing through over 2 litres of brake fluid altogether. It took a long time and a lot of fluid through each calliper before air suddenly started coming through, hence why used over 2 litres of fluid. That car now has brakes that grip after pedal just pressed, and pedal doesn't sink.

Tried the same on the C8 of course, but no air comes out, just lots of wasted brake fluid.

What would cause each separate fault? I can't understand why changing pads has caused a sinking pedal on the C8s. I undid the bleed screws before pushing pistons back in, despite not understanding how the seals can withstand hundreds of psi of back-pressure against them when pedal is pressed, but seals allegedly flip when a bit of back-pressure applied from screwing a piston in. I didn't clamp the hoses though, just opened bleed nipples.

I've read that servo faults can cause a sinking pedal, but logically that can't be from a lack of vacuum or vacuum leak as some claim. So, is the thinking that the servo doesn't stop applying vacuum and so keeps pulling the pedal down with tremendous force until the pads are dug deep into the discs and the pedal touches the floor? Or is it simply that powerful servo assistance will cause all brake pedals to sink to a degree, but it'll be much more noticeable if it is too easy for the pedal to go down because of air in the system which easily compresses, or leaking fluid or bulging hoses, etc?

More concerned by the length of pedal travel.

Thanks.
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Re: Citroen C8 Brake Pedal Excessive Travel

Post by Gibbo2286 »

Unless you have fluid leaking out of the system somewhere you most certainly have a failed/failing master cylinder, gentle pressure on the pedal is letting the fluid creep back past the piston seals.

A pedal only operating from half way down suggests that one of the master cylinder seals has completely failed and you only have half of the split circuit braking system working, probably as a diagonal pair left front - right rear or vice-versa.

I'm not sure which way they're split on your model.
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Re: Citroen C8 Brake Pedal Excessive Travel

Post by GiveMeABreak »

James, I'm assuming you may not have this procedure for C8 brake bleeding - given that is says you mustn't switch on the ignition whilst undertaking the procedures - and in your post you have mentioned doing this with the engine running. Also there are 2 procedures to follow, so I've listed it completely here in case you don't have it:

CAUTION : The use of DOT4 brake fluid for the brake circuits is compulsory.
CAUTION : During the bleeding operation : Keep the fluid topped up in the reservoir and complete it.
N.B. : Use only clean fluid free from bubbles. Avoid any introduction of impurities into the hydraulic system.

1. Bleeding the primary braking circuit
CAUTION : Do not switch on the ignition at any time during the operation.

N.B. : Bleed the primary circuit until clean, non-foaming brake fluid is obtained.

There are 2 procedures for bleeding the primary brake circuit.

1.1. Bleeding the primary braking circuit (With the bleeding device)
Bleed the circuit following the instructions for use of the equipment.

1.2. Bleeding the primary braking circuit (without brake bleeding equipment)
N.B. : Two operators are necessary.
  • Couple a transparent tube on the bleed screw .
  • Press the brake pedal slowly.
  • Open the bleed screw .
  • Hold the pedal fully down.
  • Close the bleed screw .
  • Allow the brake pedal to return naturally.
  • Repeat the operation until the fluid running out is clean and free from air bubbles.
  • Proceed in the same way for the other wheels.
N.B. : Check that the wheels rotate freely when the parking brake is released .

1.3. Checking : Bleeding the primary braking circuit

At the end of the bleed programme, check and top up, if necessary, the brake fluid level.
Check the travel of the brake pedal (no extension).
If the travel is long and spongy, repeat the bleed procedure.
Start the engine.
Check the travel of the brake pedal (no extension) ; Road test the vehicle (if necessary).
If brake pedal travel is not correct ; Bleed the secondary brake circuit.

2. Bleed the secondary brake circuit
Bleeding of the secondary braking circuit is done with a diagnostic tool.
This procedure is only carried out if the primary brake circuit has been bled.
Continue the procedure for bleeding of the hydraulic circuit as defined by the diagnostic tool.
At the end of the bleed programme, check and top up, if necessary, the brake fluid level.
Check the brake pedal travel (no extension), otherwise repeat the bleeding procedure.
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Re: Citroen C8 Brake Pedal Excessive Travel

Post by BX »

I'm with Marc on this one. I just want to add that this or a very similar procedure is required for practically any model introduced after the year 2000 regardless of manufacturer. There are sections of the ABS system which are virtually impossible to bleed without a diagnostic tool. Last one I encountered was a 2007 corolla. Sometimes even if the tool does not have a bleed function using it to activate the ABS pump and continuing the old fashioned bleeding process can work.
Last edited by BX on 06 Nov 2016, 23:08, edited 1 time in total.
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Re: Citroen C8 Brake Pedal Excessive Travel

Post by GiveMeABreak »

BX wrote: a very similar procedure is required for practically any model introduced after the year 2990 regardless of manufacturer. .
You must be Doctor Who! :-D
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Re: Citroen C8 Brake Pedal Excessive Travel

Post by uncle buck »

JamesQB wrote:.... However, this has created another issue which seems common enough for the C8 that there are quite a lot of posts on various forums but never any return to say what the solution was: the brake pedal now sinks to the floor if pressure on the pedal is maintained.
With the engine off pump the pedal until you evacuate all of the vacuum out of the servo, now does the pedal creep to the floor when you hold pressure on it?

If the pedal is firm maybe your problem is Diesel Brake Creep.... http://www.brakesint.co.uk/technical.html#techNote202


Has the car always had excessive pedal travel?

Has the master cylinder or servo been changed recently?





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Re: Citroen C8 Brake Pedal Excessive Travel

Post by BX »

Well spotted Marc. Sorry to anyone misled by the above date. I was just back from a short break in 2301 and lost track of time
GiveMeABreak wrote:
BX wrote: a very similar procedure is required for practically any model introduced after the year 2990 regardless of manufacturer. .
You must be Doctor Who! :-D
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