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Disco 2 Brake pedal problem when towing weight

Discussion in 'Land Rover Discovery' started by Jamiegreen, Sep 24, 2019.

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  1. Henry_b

    Henry_b ̿̿ ̿ ̿'̿'\̵͇̿̿\з= ( ▀ ͜͞ʖ▀) =ε/̵͇̿̿/’̿’̿ ̿ ̿̿

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    @Jamiegreen

    when does the "sinking" occur?

    After a good drive with the trailer behind?
     
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  2. Interceptorxj

    Interceptorxj Well-Known Member Full Member

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    I did and already new of it but as I stated I read from his post that this simply happens when he hooks the trailer up not from prolonged brake usage which i found very bemusing.
     
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  3. Stanleysteamer

    Stanleysteamer Well-Known Member Full Member

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    So I think we both agree it is more likely to be the trailer brakes than the towing truck brakes.
     
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  4. Jamiegreen

    Jamiegreen Well-Known Member

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    Apologies for the late reply. Ended up bleeding one more time and putting it back to work for a day to see if they were any better. They were not.
    There's no specific trailer it pulls, there's about 3 different ones, and another 2 trucks which brake them fine. Was mentioned to me that whilst braking when someone pulled out Infront the pedal was low but working, but when the abs kicked in the pedal hit the floor and had to be pumped back up. Doesn't seem to be temp related. Happens as soon as the weight is put on. But as mentioned I doubt brake fade etc would cause the pedal to drop but rather just be inefficient.
    There's only 2 parts left I could think of changing, one being the ABS pump (leaking internal seals) not sure on that one. And the other being the britpart cylinder (possibly letting fluid bypass the seal with the extra pressure)
    I have all these spare parts so it's not a big deal to try them..
    I suppose being brand new brakes it could be a slightly higher pedal without the trailer too, but seems pretty average to me.
     
  5. sierrafery

    sierrafery Well-Known Member

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    Did you try with the trailer socket unplugged too? to rule out some odd electrical inteference
     
  6. iansoady

    iansoady Active Member

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    It's hard to think how that could affect the pedal height.....
     
  7. sierrafery

    sierrafery Well-Known Member

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    That's why i said "odd" cos some backfeed through the brake pedal switch's circuit in a peculiar way can affect the management to open the IV's in the moduator and lower the pressure... even if it's not likely IMO it should be ruled out cos the symptom is odd too

    i've seen so many oddities on LRs that nothing can surprise mee
     
    Last edited: Sep 28, 2019
  8. Dippypud

    Dippypud Never Knowingly Understood Full Member

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    Brake servo problem then ?
     
  9. Stanleysteamer

    Stanleysteamer Well-Known Member Full Member

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    He could test this quickly (and illegally) by unplugging the plug and driving it a bit, including braking obviously.
    I'd give this about a 1000 to one chance of being the reason, but then I am not an electrical wizard as everybody knows!
     
  10. sierrafery

    sierrafery Well-Known Member

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    mee too but worth to be ruled out anyway
     
  11. Jamiegreen

    Jamiegreen Well-Known Member

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    I can give that a try, but the fault would have to be with the wiring on the disco as it's had more than one trailer on.
    Any thoughts on if it's worth swapping the abs pump as it's the only thing that hasn't been replaced?
    And failing that I'm still wary of that britpart cylinder.
    Haven't had the truck back yet so will do some more troubleshooting when possible. Thanks for replies
     
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  12. sierrafery

    sierrafery Well-Known Member

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    I'd blame first the britpart cylinder rather than the modulator
     
  13. Stanleysteamer

    Stanleysteamer Well-Known Member Full Member

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    +1^^^ if you've changed the trailers around it is not likely to be all of them.
     
  14. kermit_rr

    kermit_rr Green Member Full Member

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    Could it simply be air still in the abs system? If the pedal goes to the floor when the abs kicks in, it certainly sounds like air.
    I presume the abs pump needs to be energised as part of the bleed process like on a normal car? Or am i talking rubbish
     
  15. Stanleysteamer

    Stanleysteamer Well-Known Member Full Member

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    One thing I am not sure anyone has mentioned yet, could it be a flexible pipe that is ballooning under the extra pressure?
    Also, have you got someone to stand on the brakes and had a real good look around underneath at all four wheels, all pipes, all flexies and under the bonnet around mastercyclinder and ABS things? I know this is basic, and therefore you may well have done it already, but lost fluid should be obvious from the fluid reservoir.
     
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  16. Jamiegreen

    Jamiegreen Well-Known Member

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    Going to have a look at it again tomorrow. I had debated the Flexi piped expanding but wasn't sure it would cause that much of a difference. Will check fluid level tomorrow but didn't notice any loss last time. I've never had an abs pump cause this so maybe I will start with the ****part cylinder..
    And yes the Hawkeye bleed procedure forces air out of the module with the pump and I really do doubt there's any left in there, especially since I bled so much new fluid was at the wheels. I'll try and take a look at the flexis whilst someone hits the brakes. Cheers
     
  17. Rangie Disco Sportsman

    Rangie Disco Sportsman Active Member

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    For these symptoms on a general towing commercial vehicle normally it would imply a problem with either the load sensing valve or the rear brakes.
    In normal operation less brake force is applied to prevent the rear wheels locking up when more weight is imposed the valve opens up and allows more forceful braking which would mean the fault is either pressure escaping at the load sensing valve or some leak on the rear system only becomes more apparent to be noticed at the pedal under this higher pressure braking.

    As far as I'm aware though the D2 doesn't have a load sensing valve although I would have thought it ought to have some system fitted given the significant axle load variation between a single driver and all seven seats being filled so don't know if it does it by using the ABS system to modulate brake force between the front and rear axles by something like measuring the pressure in the rear airbags to assess load then altering brake force to the rear axle but I would be very surprised if it was as sophisticated as that but the reason vehicles on air suspension can't use traditional brake load sensing valves is they are activated by steel springs sinking to indicate load whereas on an air sprung vehicle it remains level the whole time.

    I would have thought though if you drove on some loose gravel and locked up a few times and the ABS worked and the pedal went hard and didn't sink that ought to mean everything is OK and that side of things though.
     
  18. Jamiegreen

    Jamiegreen Well-Known Member

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    As far as im aware the slabs ECU uses the ABS module to monitor the pressure sent to each brake/axle and adjusts it accordingly, I assume it measures how much pressure is needed to brake each axle and compares it to the speed reduction to determine weight load, although just a guess for a very complicated system.
     
  19. sierrafery

    sierrafery Well-Known Member

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    It does not measure any pressure as there is no pressure sensor in the system, the algorythm is based on a deceleration value induced by the wheel speed sensor's inputs on which is calculated the VSS(vehicle speed signal) so if the vehicle doesnt decelerate enough according to the calibration of the system the ECU will manage the modulator's pump and inlet/outlet valves as to increase/decrease pressure untill the deceleration threshold is acceptable for it's stored(mapped) values.... it's a very complicated system... took me about 2 years of hard study to understand it and that's why i'm "obsessed" about tyre dimensions too... belive me or not, i'm sure that there are smarter guys than me out there, so called "specialists"... the difference is that i provide well documented informations for free not for profit, i can give you a small part of what's involved, i've studied at least 10 doc more and corroborated the info but the attached doc is quite relevant and if somebody is determined enough to read it at least twice like i did will understand
     

    Attached Files:

    Last edited: Oct 7, 2019
  20. Rangie Disco Sportsman

    Rangie Disco Sportsman Active Member

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    Looking at the PDF linked above it sounds like they have replaced a load sensing valve with an EBD system on the ABS modulator - not great in my general opinion and part of the reason Discos and P38s often seem to be weaker braking that pickup trucks and vans loaded to similar weights despite seemingly having inferior drums on the rear but that’s another discussion.

    From a quick read it works by always making sure the rear brakes do not lock before the fronts and apparently you get no noise when it is in operation only a slight pulsation at the pedal, less than experienced with ABS actuating. With a trailer hitched, despite the suspension levelling things up there’ll still be loads less weight on the front wheels under braking so the fronts will potentially lock up much much earlier at which point thr system will be happy to send full force to the rears and this is probably the point the pedal then sinks to the floor. Can only really guess as to the cause as don’t know if a modulator/pump/valve/solenoid/electrical issue could cause this or a more traditional ‘hard’ fault with the rear brakes that only manifests itself under full brake pressure but to be honest if there’s no sign of brake fluid loss it’s got to be a fault with letting the fluid back out by outlet valves in the system.
     
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