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Mini Disc Problems

Discussion in '40- & 55-Series Tech' started by Cruiser4Randy, Jun 17, 2003.

  1. Cruiser4Randy

    Cruiser4Randy

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    I am completing a mini truck disc brake conversion on my ’72 FJ40 and have run into a problem. The problem is in the rear brakes. They don’t lock up :dunno:. I changed the stock MC to a unit out of a ’83 Celica. That car should have had disc front and drum rear. I have a feeling that the problem lies in the lack of a residual valve for the rear. I checked the old Celica MC and could not find any residual valves. Could there have been a valve that was place “inline” to give the rear drums the necessary line pressure? If so, is there a way to put a residual valve “inline” to solve the problem. If all else fails, I can go back to the original MC and just remove the valve out of the front :G.

    Randy <><
    aka. webmaster
    ‘72 FJ40, stock F engine, LC 4 speed,
    York, No-Slip(rear), Lock-Right(front),
    Mini Disc Front, Saginaw P/S.
     
  2. fj55-100

    fj55-100 SILVER Star

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    randy i ran the same combo of parts in my 72 with no problem, but summit racing sells a residual valve , over time i swiched my master for a 1 ton toyota pu, the booster is the small fat one very similar to the later fj40 one only at a tenth of the cost
     
  3. 73lndcrsr

    73lndcrsr SILVER Star

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    If I recall the celica brakes are signigicantly smaller than the ones on the truck or cruiser. I know the weight difference is great. You may just not have enough volume to make the rear drums lock up.
     
  4. Cruiser4Randy

    Cruiser4Randy

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    My research showed that even though the Celica was significantly smaller and lighter, the MC still produced the same volume as the mini truck. Both bores were 7/8". The larger bore (15/16") was used on other vehicles. I am almost sure that the MC I selected was from a Celica with four wheel disc brakes. I am going to test it further today and if I don't feel comfortable with the braking, I well go back to the original MC with the residual valve removed from the front line.
     
  5. 73lndcrsr

    73lndcrsr SILVER Star

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    That is good information to know. I would not have thought the celica had that big of a MC.
     
  6. Cruiser4Randy

    Cruiser4Randy

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    Well today I jumped in the truck and went for a good test drive. I didn't get a half mile from the house when I noticed the brakes were locking up. At first I thought it was the rear brakes but it turned out to be the fronts! I limped it home and switched out the MC back to the original for my '72. I did remove the residual valve from the front line. After bleeding the system, I again ventured out on the road. This time everything worked perfect. :doh: The moral of this story seems to be: Don't use a master cylinder from a '83 Celica! My thanks to all of you who responded.