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360 brake calipers blocking after ABS unit swap

Discussion in 'Technical Q&A' started by ManuFromParis, Oct 29, 2020.

  1. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    Hi guys,

    I had a leaking ABS Unit so I swapped with a 2nd hand one, supposedly taken from a car that was damaged in a barn roof collapse, so in working condition.

    I must say that before I noticed that my old unit was leaking, I swapped all disks and pads but very hard breaking remained tricky.

    Strangely, the previous leak was small and never let air in the system, that's why it was noticable only from under side.

    I did not have any lights on dashboard or error codes about braking.

    Anyway, I swapped the ABS units yesterday and it was tricky to bleed all air out with the traditionnal bleed procedure, but now the brake pedal is firm but the system remains under pressure, meaning everytime I touch the brake pedal, it brakes but the calipers don't open back totally, and quite quickly it becomes impossible to move unless I open the bleed valves on both sides to release some pressure.

    This release of pressure could to be done in both front wheels or both rear wheels, as the system is divided and linked (see picture attached).

    But the cure doesn't last : if I press 5 or 6 times, even slightly, it blocks back.

    I let the calipers bleeding valves open for an hour, hoping that remaining pressure would go away, but without success.

    Next Week-End I'll perform another bleeding using the AUTEL MS908 Pro from a friend, but I don't understand why this pressure problem happens, since the braking system should work even if the ABS Unit is faulty.

    Any idea or suggestions ?

     

    Attached Files:

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  3. wmuno

    wmuno Karting
    Silver Subscribed

    Dec 24, 2007
    111
    Wilmette, Illinois
    Full Name:
    Bill Muno
    I had the same problem on an older Ferrari. The original flexible rubber brake lines expanded on the inside acting as a one-way check valve. From the outside they all appeared normal. Replacing all 4 of them solved my problem.
     
  4. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    I doubt that this would happen suddenly after such an Open/Close operation on the system.
    I've performed a full bleeding when replacing the disks and pads, everything was normal, and the car is only 15 years old.
     
  5. virendra

    virendra Karting
    Silver Subscribed

    Jun 14, 2008
    226
    Hong Kong
    Full Name:
    virendra anand
    Will disconnecting the new ABS assembly power help? You can then establish if the fault is due to a sticky solenoid or due to a hydraulic lock. You are assuming that the old part is perfect. What if it has any debris in it?
     
  6. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    You mean, pulling the ABS plug ?
    I haven't tried that, expecting the car to enter in fault, but I'll try that right away.

    The 2nd hand part was unfitted by a pro and had plastic plugs on all 6 fittings, so yes I am assuming it was "clean" inside.
    Would a debris hold the pressure on both sides at the same time ?
     
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  8. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    The fact is that during the bleeding process, just after the swap, the contact never was "ON".
    Eventually I tried the pedal with contact and engine ON.
    I'm not sure if the calipers were blocked before or after I started the car to check that the pedal remained firm.
     
  9. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    I just performed another test :
    without setting the contact ON, I pushed the brake pedal 5 / 6 times and the calipers got into pressure right away.
    So t's definitely somthing holding pressure within the unit.
    :(
     
  10. Ferrari Tech

    Ferrari Tech Formula Junior

    Mar 5, 2010
    639
    Georgia
    Full Name:
    Wade Williams
    Start with the rubber lines. I have serviced cars and performed a brake flush and the lines will block. Using the brakes moves very little fluid. Bleeding the brakes moves a lot of fluid and I suspect it does cause the rubber lines to "fold" on the inside and block fluid.
    It is good maintenance as well, so you are not wasting money to try it.
     
  11. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    Your hypothesis would be that the 4 rubber lines failed simultaneously ?
    I think it would be very unlikely, but thanks for the suggestion.
    I'll probably look into that after I performed the "electronic" bleeding, as they say in the service manual.
     
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  13. eric355

    eric355 Formula Junior
    Silver Subscribed

    Nov 30, 2005
    977
    Toulouse (France)
    Full Name:
    Eric DECOUX
    Bonjour Emmanuel.

    Are you able to release the pressure if you slightly unscrew the pipes on the master cylinder ?
     
  14. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    Salut Eric,
    Yes, I can release the pressure from the MC valves, and I confirm that both valves needs to be opened to free the car totally.
    On each valve, around 3cm of fluid goes out into the bleeding tube.
     
  15. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    I just talked to Eric and we now are quite convinced that the problem comes from the MC that does not allow the pressure to go back into the feeding lines and reservoir, so I guess my option is now to unfit this guy and hope I can clean it up...

    Stay tuned and cross fingers for me !
     
  16. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul

    Attached Files:

  17. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
  18. yelcab

    yelcab F1 World Champ
    Consultant

    Nov 29, 2001
    10,521
    San Carlos, CA
    Full Name:
    Mitchell Le
    I don't think the fault is in your master cylinder. The fault is probably in the second hand ABS unit you swapped in. If you look at the diagram, there are valves in the way between the MC and the Calipers. Those valves, or one of them, is blocking the path for the fluid to go back to the master cylinder.

    Do the ABS bleed with your friend's computer unit, and if that does not work, find another ABS unit. Either rebuilt one or new one is better than an unknown used one.
     
  19. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    Well, the guy who sold me the unit is absolutely positive that he drove the car before taking it apart, and the brakes were working. He sent me the unit the same day.
    The pressure release on the MC showed that the pressure got back to the MC, but not further to the fluid tank.
    Why is that, I don't know, but it would make some sense that the MC was faulty and that it doesn't show on inside parts.

    We'll know this week, when i'll fit another MC.
    Except if it is my bleeding procedure that created an over pressure, but I wonder how that would happen...
     
  20. eric355

    eric355 Formula Junior
    Silver Subscribed

    Nov 30, 2005
    977
    Toulouse (France)
    Full Name:
    Eric DECOUX
    I know we discussed that but even a perfect MC won't allow pressure release if it can't go back to its rest position. A few tenth of mm can be enough to mask the compensation hole and block the path back to the tank.
    May be something happened during bleeding by the pedal pumping method. I personnaly don't like this method and do prefer the pressure bleeding which is harmless for the MC.
     
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  21. radlu

    radlu Formula Junior

    Jun 6, 2005
    342
    Great work!
     
  22. ManuFromParis

    ManuFromParis Rookie

    Apr 3, 2020
    47
    Paris
    Full Name:
    Emmanuel Paul
    Eventually I managed to get the brakes working properly, and ther was several problems hiding one's another :

    When I performed the "electronic" bleeding with the AUTEL Pro, some dirt was released and it blocked the front right external side of the caliper. I had to remove the small line at bottom of the caliper and clean it.

    I replaced the MC but the blocking remained.

    During the whole process, with the help of a friend pushing the brake pedal, somehow the brake booster was damaged and that's the reason why the brakes were blocking : the MC could not get back to it's rest position, then let the fluid get back to the tank.

    So I had to replace that booster and everything was solved, with both MCs.

    Note for later : you can bleed the system pushing on the brake pedal, but never get to the end of the pedal travel !
     
  23. lkstaack

    lkstaack Rookie
    Silver Subscribed

    Dec 9, 2020
    43
    San Diego
    Full Name:
    Lars
    Actually, that's good advice for pump bleeding any older vehicle. The master cylinder piston can wear a slight ridge at the end of it's travel through normal use. The piston can easily extend past this ridge during pump bleeding, damaging the piston seal.
     

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