Ass dropping under braking
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  1. #1
    Fellow Frogger! gsmack's Avatar
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    Default Ass dropping under braking

    Hi All
    I have a bit of a problem with my BX, when I apply the brakes the back of the car drops suddenly quite often.

    I have bled all the brakes, and am about to go and bleed the control valve, does anyone have any other ideas or suggestions.

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    It improves when there is load in the back, but that doesn't completely solve it.

    The brakes feel fine and work well, it's just the dropping issue.

    Thanks

    G

  2. #2
    Fellow Frogger! XM Mechanic's Avatar
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    They tend to do that anyway as they use fluid from the rear suspension to apply the brakes. If it is really bad then you could suspect a internally leaky doseur valve. ie brake valve. You can try a new or overhauled one of those, but I wont guarantee it will fix it.

    Why it does it less with weight in the back is there would be more pressure in there to start with.

    Let us know how you go.
    Regards,

    Garth.


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  3. #3
    JBN
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    Another thing that you can do is go through the Citroen aerobics routine a few times (ie full height to minimum height, 3 - 5 cycles.

    I am fortunate in that the underground parking in my townhouse complex is on a slope. Along the edges, just in front of the individual garage doors, there are some big rounded humps (to accomodate the horizontal garage floor in relation to the sloping driveway). It is amazing how driving down slowly with the left side suspension going through its large range of movement, before reversing and doing the same on th right side, improves the ride and operation of the hydraulic suspension.

    Citroens like to be chastised. A good hard drive over undulating road surface gets rid of the cobwebs. My wife's sedate driving puts the suspension into a comatose state.

    John

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    Contented Peugeot Driver addo's Avatar
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    Mine does this; I have observed wear in the doseur but not quantified the leakback rate. Also I believe that low pressure in the rear spheres contributes.

    The other unquantified detail I have in my sights, is regulator pressures; if the cutoff pressure was too low it might exacerbate the above items.

  5. #5
    Fellow Frogger! gsmack's Avatar
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    Addo...what are you doing on Aussie frogs...? :-P

  6. #6
    Too many posts! JohnW's Avatar
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    Quote Originally Posted by gsmack View Post
    Hi All
    I have a bit of a problem with my BX, when I apply the brakes the back of the car drops suddenly quite often.

    I have bled all the brakes, and am about to go and bleed the control valve, does anyone have any other ideas or suggestions.

    It improves when there is load in the back, but that doesn't completely solve it.

    The brakes feel fine and work well, it's just the dropping issue.

    Thanks

    G
    All three of our hydropneumatic cars do this to different extents and rates, depending for how long the brakes are being applied. For example, if I sit on the brake at lights instead of using the handbrake, they all drop practically to the bump stops, and it can be a bit sudden in the BX.

    Are your sphere pressures OK?

    Is it causing you problems or more aquestion of "do I have a problem?"
    JohnW

    Renault 4CV 1951
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  7. #7
    Fellow Frogger! gsmack's Avatar
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    Hi All

    Thanks for those thoughts. I need to get it fixed so I can get a RWC for the car, otherwise I feel it is unlikely to pass.

    Bleeding the control valve appeared to improve it, and I hadn't actually checked the pressure in my accumulator since getting the car, so that is off to get filled.

    After that is done, I'll see what it is like.

    Sadly it looks like I have a leak from my front height controller though

  8. #8
    1000+ Posts michaelr's Avatar
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    Just a question.... does the rear of the car drop if you use the brakes whilst stationary, or only at speed? If it happens when standing then it must be a hydraulic problem but if only at speed then read on.

    In a DS the rear drops partly because the braking takes a little pressure from the rear circuit but mostly because of the geometry of a trailing arm rear suspension. If the BX also has a similar trailing arm rear, and the car has only "dropped" since you bled the brakes, then maybe it is just a suspension characteristic not previously noticed. My first ID19 never dropped its tail until I un-seized the rear brakes... I had driven it for almost a year on just front brakes without realising!
    Michael
    Member, Citroen Car Club NSW

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  9. #9
    Too many posts! JohnW's Avatar
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    Quote Originally Posted by michaelr View Post
    Just a question.... does the rear of the car drop if you use the brakes whilst stationary, or only at speed? If it happens when standing then it must be a hydraulic problem but if only at speed then read on.

    In a DS the rear drops partly because the braking takes a little pressure from the rear circuit but mostly because of the geometry of a trailing arm rear suspension. If the BX also has a similar trailing arm rear, and the car has only "dropped" since you bled the brakes, then maybe it is just a suspension characteristic not previously noticed. My first ID19 never dropped its tail until I un-seized the rear brakes... I had driven it for almost a year on just front brakes without realising!
    I don't think it makes any difference other than the engine speed, i.e. hydraulic pump delivery rate, means you get the effect particularly if you leave your foot pushing the brakes when stationary. The hydraulic system doesn't know whether or not the wheels are turning. The rear end drops with all ours if you hold the pedal down for up to a minute when stationary. I wonder if the offending BX rear end has flat spheres.

    Re rear brakes, I wonder why they bothered with the CX. I guess legal expectations, but they don't do much.
    JohnW

    Renault 4CV 1951
    Renault R8 1965
    Renault Scenic 2005 (wife's)
    Renault Scenic 2007 (mine)
    Renault Scenic 2006 (daughter's)
    CitroŽn CX Pallas 1980

    National Co-ordinator, Renault 4CV Register of Australia

  10. #10
    1000+ Posts michaelr's Avatar
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    Quote Originally Posted by JohnW View Post
    I don't think it makes any difference other than the engine speed, i.e. hydraulic pump delivery rate, means you get the effect particularly if you leave your foot pushing the brakes when stationary. The hydraulic system doesn't know whether or not the wheels are turning.
    Nope, the hydraulic system doesn't know if the wheels are turning, but hopefully the driver knows !

    The thing is that the rear drops on a trailing arm suspension as a dynamic reaction to applying the brakes. If gsmack's car only drops when the brakes are applied at speed then this could be the explanation. (Because the hydraulic system does not know the wheels are turning ! )
    Michael
    Member, Citroen Car Club NSW

    DS23 Pallas 5 sp. "Francoise" , BX19TRi Auto "Jacques Dutronc" , Teardrop Trailer "The Toad", BMW R65 "Rosamund"
    In the past: Renault 750, Dauphine, R4, R8, R10, Peugeot 504 Familiale, ID 19 (x2), Safari (x2)

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