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Tandem MC Types & Problems

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  • Brakes: Tandem MC Types & Problems

    At the IM in Mansfield I saw a drum braked Hawk, with tandem MC, mounted in a Turner bracket. Not unusual, but I'd never seen a MC like that one, which had a large front reservoir, and small rear. The car had dragging brakes, so I first looked at the MC rod, and it was three turns past zero free play. In other words, was pushing the piston into the MC by three full turns. I backed it out till about 1/8" to 1/4" of free play at the pedal, but then the pedal went to the floor when pushed. It would only come back to regular height when extended three turns again. A fellow SDCer said his Studebaker mounted Mitsubishi MC required rod preload in order to have sufficient pedal, and apparently this one did too. Only other possibility I can think is if the front RPV was absent. Without an RPV, the shoe springs could push so much fluid back into the MC, that a single stroke of the pedal would not reapply the brakes. Other than that, no clue.

    At any rate, we fixed the original problem of brakes dragging with new hoses at the HV. I also adjusted all shoes. Later, when we test drove it several miles, the owner said pedal height was getting taller. I figured the rod preload was causing the brakes to drag slightly, so we stopped and turned the rod in by one turn, but then the pedal nearly went to the floor again. So I extended the rod back, one flat at a time, till five of the six flats were returned (5/6th of one turn). This seemed to be a fine line where, any more extension and the brakes began to overheat, but any less and the pedal began to drop.

    I have always needed the 1/8" to 1/4" pedal free play to avoid dragging the brakes. But apparently not all MCs are created equal, or possibly that car is missing a front RPV. Still scratching my head over that one.
    Last edited by JoeHall; 09-15-2019, 05:10 PM.

  • #2
    That is interesting information. At some point, I would like to upgrade to a dual MC, and front disc. I have considered foregoing authenticity, and installing hanging pedal and firewall mounted MC with booster. That would require moving the battery. For now the four drum manual setup works fine, but I haven't driven it in heavy traffic, that may require more of a panic stop. I'll do some reading on MCs and see what I can find on actuator rod free play. Thanks for posting.

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    • #3
      tsen -

      Why go to all the trouble, just leave all under the floor..! Keep under the hood clean.
      I've got two 54 wagons. The factory pedal length and pivots are perfect for a .937" (15/16") or 1.00" dia. piston (dual), master cylinder (Wilwood aluminum).
      I used the 1.00" piston, no booster, four wheel disc, stops with a two toe push, near perfect.
      Would not even think of moving everything, waste of time, effort and money.

      Mike

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      • #4
        Thanks Mike. I would rather leave under the hood the way it is. I thought maybe disc brake calipers would require some sort of booster.

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        • #5
          This sure sounds like a case of no 2# residual pressure valve in the front disc brake circuit. Joe when you said as soon as you put some clearance in the push rod the pedal goes to the floor. when you adjusted it to have virtually no clearance you had pedal. What could be happening here is when you back off the push rod the fluid from the front calipers is allowed to drain back to the master as the calipers are higher than the master. Next brake application pedal goes to the floor. As soon as you take up the clearance of the push rod the fluid can no longer drain back hence you retain your pedal height. Of course the brakes now will drag. All this is assuming the master cylinder of this system is lower than the calipers.
          Frank van Doorn
          Omaha, Ne.
          1962 GT Hawk 289 4 speed
          1941 Champion streetrod, R-2 Powered, GM 200-4R trans.
          1952 V-8 232 Commander State "Starliner" hardtop OD

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          • #6
            41 Frank the car has shoes, front & rear, so the RPV should be 6 PSI, which just gives more credence to what you said. I thought that too, after having slept on it. It's like the MC was set up for disc brakes (original RPV removed from the MC), but no in line RPV installed, front or rear, then installed in a drum braked car. By extending the push rod 3 turns, it keeps the piston pushed forward into the MC about 1/8", so the fluid return port is blocked off, I am guessing.

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            • #7
              Originally posted by Mike Van Veghten View Post
              tsen -
              Why go to all the trouble, just leave all under the floor..! Keep under the hood clean.
              Would not even think of moving everything, waste of time, effort and money.
              Mike
              Click image for larger version

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              It looks pretty clean to me, not to mention how much easier it is to work with. Doesn't take too much time to change, if you don't get real fancy with stuff. Cost was $42 for a new chrome 7" dual booster and $26 for a new '69 Corvette (manual brake*) master cylinder. Scrap steel sheet and Lark pedal was free. The addition of a booster to even the stock drum brakes makes a huge difference in their effectiveness.
              (*Manual brake master is a 1"x1" bore, power brake master is dual bore 1"x 1 1/8")

              Originally posted by JoeHall View Post
              By extending the push rod 3 turns, it keeps the piston pushed forward into the MC about 1/8", so the fluid return port is blocked off, I am guessing.
              That's what I was thinking too.

              sigpic

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