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  1. #1
    CB750 Member Stoney's Avatar
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    Extremely weak front brake...

    Hi guys. I have a 1978 cb750k which despite my best efforts, is refusing to brake effectively (front brake only, rear is great).
    In my troubleshooting journey I have...

    • Replaced both brake pads with new ones
    • Replaced brake fluid
    • Tested system with a brand-new master-cylinder
    • Bled the system about a dozen times (albeit without the proper equipment, although I'm fairly sure there is no more air in there)
    • Dismantled and cleaned out caliper, there is some light pitting on the piston but it appears to move in and out (maybe a couple of mm?) well with application of brake lever
    • The lines have been replaced with stainless steel aftermarket ones
    • Gapped the floating brake pad as-per manual specification

    I'm starting to wonder if the rotor might be the issue, although it appears to be in perfect condition with no scoring.
    I will be replacing the piston and seals next.
    I'm also looking to splash out on a proper tool for removing air from the system as I know that this can be a huge problem and I understand that the old school method (pump lever, hold lever, loosen bleed nipple, tighten bleed nipple, repeat) is probably not effective enough to guarantee results.

    If anyone can either re-affirm one of my next planned steps or suggest something else I might be overlooking that'd be great. The bike is off the road right now because of this problem and I'm yet to ride it! Very frustrating.

    Cheers now.

  2. #2
    CB750 Addict dan1951's Avatar
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    Probably still air in the system, Brake lever spongy or solid feel? No need to change the rotor unless it pulsates or is under thickness spec. tolerances. I would still rebuild the caliper.

  3. #3
    CB750 Member Stoney's Avatar
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    Quote Originally Posted by dan1951 View Post
    Probably still air in the system, Brake lever spongy or solid feel? No need to change the rotor unless it pulsates or is under thickness spec. tolerances. I would still rebuild the caliper.
    Thanks for ruling out the rotor man. Lever does feel spongy right now, I've managed to get it fairly firm before now but it only marginally improved the braking power. It's probably spongy right now because I just fitted a new mc and got air into the works again! Can you recommend a good bleeding tool?

    Thanks

  4. #4
    CB750 Addict dan1951's Avatar
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    I purchased a tool made by mity-vac it comes with awide array of fittings and hoses, I neglected to ask if You adjusted the caliper bolt? It will move the caliper in or out.

  5. #5
    CB750 Member Stoney's Avatar
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    Quote Originally Posted by dan1951 View Post
    I purchased a tool made by mity-vac it comes with awide array of fittings and hoses, I neglected to ask if You adjusted the caliper bolt? It will move the caliper in or out.
    Cool, I'll look into that. The adjustment screw that changes the position of the caliper arm was set so that I had a .008 gap between the static pad and the rotor, when I pump the brake back up to full pressure that gap closes and does not reopen.

  6. #6
    CB750 Addict dan1951's Avatar
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    To properly adjust it You need to have the front wheel off the ground and spin it. Turn the adjustment screw in until there is a slight drag on the rotor then slowly back it off until wheel spins free again then another 1/4 turn out and tighten the jam lock nut, And it should be fine.

  7. #7
    CB750 Member Stoney's Avatar
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    Managed to get this sorted out eventually! I had several problems:
    First was that my master cylinder seals were shot (allowing air back into the system). I was going to replace them but ended up just forking out on a brand new master cylinder from Dave Silver spares.
    Second was the the groove inside the caliper which the little rubber ring sits in was FULL of gunk. I removed the old ring, scraped all the filth from the inside of the groove with the edge of a small allen key and then replaced the ring with a new one. I now have full braking power and the piston is retracting correctly too. Thanks for the help Dan. Now to iron out some other problems!

  8. #8
    CB750 Addict dan1951's Avatar
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    Glad to see You got Your problem resolved Stoney, Good to hear that.

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