Question about Honda CB 750 Motorcycles

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1977 Honda CB750 Front Master Cylinder

Greetings,

I recently ordered & received several items from you one of witch was the front brake master cylinder rebuild kit. I have attached a photo of the original one that I disassembled…I normally take photos to insure I rebuild properly. In any event I have completed the rebuild and have thoroughly bled the brake lines; there is no air in the lines for sure at this point. I would like to add that I also cleaned the master cylinder itself and carefully inspected the bore, there was no scaring or pitting; just clean and smooth. Prior to rebuild I totally lubricated the rebuild parts & the master cylinder bore with brake fluid then put it all back together. The rebuild went very smoothly. Oops I forgot to add that the calipers themselves are free and work nicely.

Now for my problem; I can’t seem to build any pressure in the in the system, the brake lever remains spongy. The calipers initially expand outward making contact with the disc put then retract as if I had let the brake lever go when I actually still have it compressed all the way to the handlebar grip.

Additionally there are no leaks anywhere in the braking system. My motorcycle only has a little over 13,000 miles on it, it sat for a long time and the reason for the attempted rebuild was that the original was leaking. Can you help me please, what the heck have I done improperly? Thank you for your time.

Best Regards,

Larry Quickel

Posted by on

  • l_quickel Sep 05, 2009

    To: adv_alexande

    As I memtioned I purchased a new rebuild kit which included the primary seal. I have noted, maybe too late, that there is more than one master cylinder bore/barrel diameter; therefore my conclusion is that I received the wrong kit. Of course when ordering the kit I specified the year, model, and frame number. Apparently I've wasted my money...when I contacted them they said they could not me of help unless my cycle was in their shop, they're in Wisconsin & I'm in North Carolina. I've not been able to located anyone that sells just the individual parts, only the entire master cylinder rebuild kit.

  • Ned C Cook
    Ned C Cook May 11, 2010

    It's terriblr how the less lines you have, the more problems there are. First, how did you bleed the brakes/?

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Larry.. the symptoms you describe are exactly what would happens when your primary seal fails, the purpose of the primary seal is to generate and hold pressure, it is located just under the little hole in the reservoir and is a little rubber seal that pushes the fluid down the line and when it fails the fluid leaks by the seal making the brakes feel spongy... replace the seal and you should have your brakes back

Posted on Sep 04, 2009

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