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Thread: test for a faulty SMC?

  1. #1
    Dccoleman's Avatar
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    test for a faulty SMC?

    After pulling the bike from winter storage and mounting new tires, I noticed the rear brake dragging. I can't find specific answers from searching the forum. If they exist, I apologize in advance. My question is twofold:

    1. Is there a surefire test to diagnose a failing secondary master cylinder (SMC)?
    2. Can the new version of the SMC issued in '09 fit my 2004 ST1300A?

    Here is some background info if it is helpful for diagnosis. I have cleaned the pistons externally with the wheel off to remove any road debris. I coated the shiny pistons with brake fluid before pressing them back into the caliper. I was able to press the center piston back into the caliper with hand pressure. I am not able to push the remaining two pistons (operated by the SMC) without c-clamps. The front and rear pads were replaced in September, 2014 at 23,000 miles when the fluid was also replaced. Current mileage is 34,000.

    Now preparing for information download

  2. #2
    Al Gully's Avatar
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    Re: test for a faulty SMC?

    Here is a great article about testing the brakes.
    I read it once a year as it has so much information on our braking system.
    Thanks John for such a great article.
    Also, I would go out and ride it around the block a few times as a bit of corrosion on the rotors might cause a slight drag if the bike has been sitting for some time. I have a infra red heat gun to check the rotors to see if one or the other is developing more heat.

    http://www.st-owners.com/forums/cont...g-the-Pitfalls


    Al
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  3. #3

    Re: test for a faulty SMC?

    Waiting by the white courtesy phone

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  5. #4
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    Dccoleman's Avatar
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    Re: test for a faulty SMC?

    Quote Originally Posted by Igofar View Post
    Waiting by the white courtesy phone
    message sent

  6. #5
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    Re: test for a faulty SMC?

    Many thanks to Larry (Igofar) who spent an hour or so on the phone helping me with my dragging rear brake. Thanks to him I feel confident I have a failing SMC.

    I should not be able to engage the rear outer cylinders by pressing the foot pedal alone while the bike is stationary. Larry surmises the SMC piston is stuck, allowing the brake pedal to charge fluid through the SMC all the way to those two cylinders in the rear. He also explained how the SMC bore can often get out of round over time. As I understand it, this is why a new piston from a rebuild kit will not always work. I have chosen to replace the SMC assembly which includes a new piston and bore rather than running the risk that a new piston alone from the rebuild kit wouldn't work properly in my existing SMC bore.

    I ordered the SMC assembly, Honda P/N 06454-MCS-G03 from discounthondaparts.com ($133.55 shipped) along with four crush washers (Honda P/N 90545-300-000 $10) and a check valve bleeder from motion pro ($11 from Amazon). I'll update once the parts have been installed and I've tested the system.

    Thanks to everyone. I hope this info helps the next guy.

    David

  7. #6
    Be Thou My Vision dduelin's Avatar
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    Re: test for a faulty SMC?

    I just checked my ST1300 whose brakes are in excellent working order. The rear wheel can be turned about 2 turns with a quick press of the foot. I then gently moved the rear outboard pad against all three pistons to gain approximately 2 mm clearance between all three pistons and the pad. I pressed the rear brake pedal hard once. Checking the piston clearances I had just opened - after pressing and releasing the foot pedal all three pistons press against the pad, removing the 2 mm clearance in all three, foot brake alone. I checked the rear wheel again and get about 2 turns. Rear caliper is operating normally.

    Then I checked the SMC for free play against the fork leg with a digital caliper. The left front caliper can be pressed about 2 mm forward thus actuating the SMC. The caliper has free movement back and forth this 2 mm upon release. SMC is free and clear. Check rear brake again - still get two turns of rear wheel. I laid a blanket down on the floor adjacent to the left side of the bike where I could press the left front caliper against the fork leg, thus compressing the SMC again. With the bike on the centerstand and holding pressure against the SMC I try and turn the rear wheel with my foot and am not able to. SMC is pressing the rear pads against the disk. I release the SMC and the rear wheel turns freely. SMC is working normally in this test sequence.
    Last edited by dduelin; 06-18-2017 at 04:45 AM. Reason: edit to add 'With the bike on the centerstand'


    Dave

    Honda ST1300
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  8. #7
    Steven631's Avatar
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    Re: test for a faulty SMC?

    In the passed, I had to learn the SMC problem too. I knew my SMC was failing, due to the burn smell of brakes, when stopped.
    The tricky part of this repair, is the bleeding of the lines.
    I had to do my bleeding procedure 3 times before I felt I did it right.
    I'M HERE TO RIDE________________

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  10. #8
    Catdriver's Avatar
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    Re: test for a faulty SMC?

    Quote Originally Posted by Dccoleman View Post
    Many thanks to Larry (Igofar) who spent an hour or so on the phone helping me with my dragging rear brake. Thanks to him I feel confident I have a failing SMC.

    I should not be able to engage the rear outer cylinders by pressing the foot pedal alone while the bike is stationary. Larry surmises the SMC piston is stuck, allowing the brake pedal to charge fluid through the SMC all the way to those two cylinders in the rear. He also explained how the SMC bore can often get out of round over time. As I understand it, this is why a new piston from a rebuild kit will not always work. I have chosen to replace the SMC assembly which includes a new piston and bore rather than running the risk that a new piston alone from the rebuild kit wouldn't work properly in my existing SMC bore.

    I ordered the SMC assembly, Honda P/N 06454-MCS-G03 from discounthondaparts.com ($133.55 shipped) along with four crush washers (Honda P/N 90545-300-000 $10) and a check valve bleeder from motion pro ($11 from Amazon). I'll update once the parts have been installed and I've tested the system.

    Thanks to everyone. I hope this info helps the next guy.

    David
    IgoFar is the man! It is truly great to have people like him that have such a strong passion for our bikes and the willingness to freely give their knowledge to interested members!

  11. #9

    Re: test for a faulty SMC?

    I wish I had a nickel for every time I heard someone say "my brakes are in excellent working condition". I have at least three failed SMC units sitting on the workbench as I type this. To be fair, one person didn't think his brakes were working correctly (he was right) the other two insisted that the brakes were fine, and working perfectly. Once they were removed, opened up, and inspected, the damage (wear) was plain to see. After new parts were installed, the owners could not believe how well their brakes worked again, and even commented on how they had no idea how bad they had gotten over the last year or two since the brake system was flushed or serviced.
    Some folks claim to have bikes that their clips never break, their hoses never leak, parts never fail, secondary master cylinders last for 200 thousand miles without service, and their suspension is dialed in good enough to be a grand prix racer, but alas, for the rest of us mere mortals, we should service our bikes often, replace coolant hoses every few years, flush our brakes, and not just assume that are brakes are in excellent working condition just because they have gotten use to how it operates.
    when a person suspects that his SMC is causing brake issues, and the part is 13 years old, why would anyone stir the pot and suggest that it may be working normally? I think it was wise of our OP to replace the entire unit, to err on the side of caution since these bikes are prone to failures in this area.

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  13. #10

    Re: test for a faulty SMC?

    I may have misunderstood Dave's response in post # 6 above but I thought he was writing that the results he described are normal when the brake system is subjected to the test procedure that he outlined. I didn't understand him to say that the OP's SMC condition was normal. I took it to be him responding to the OP's request for an SMC test procedure by describing his procedure and the expected outcomes. As the brakes are an important mechanical system and the SMC is known to be a weak point I would like to know if I missed something?
    Andrew


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