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  1. #6031 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru barnone's Avatar
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    6-54.jpg

    Could there be a problem with the master cylinder check valve which I assume the DF has? Maybe a bit of something that does not allow the valve to seal?
    I would bleed/flush the fluid first to see if that would cure the problem. If that didn't work then disassemble the master cylinder to look at the check valve.
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  2. #6032 Re: Dong Fang DF 250 RTB Bobber Information 
    C-Moto Guru ben2go's Avatar
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    The check valve holds about 2psi in the system to keep the pistons from returning when the lever is released but it's not enough pressure to cause brake drag. Over on chinariders, there was a member that had a similar problem. He removed the master cylinder and did an old fashion bench bleed. During the bleed, some hardened chunky brake fluid debris came out of the hole that allows fluid to travel down through to the piston. You should be able to disassemble and clean the master cylinder without dmaging any seals. I have done it on my Jap bikes with no problems.
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  3. #6033 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru axa's Avatar
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    ya ill clean it out before killing myself looking for a rebuild kit...

    EDIT:
    cleaned it out. nothing to be found.
    no definitive measurements but the piston/seals seemed to fit a bit loose in the bore for a high pressure sealing brake system.
    well i guess the definitive measurement would be almost not enough pressure to push fluid out the bleeder so... need new seals.

    but it seems the rebuild kits are a bit confusing as each seems to be built a bit differently.
    Does anyone have a part number on a kit?

    Thing is entire new master cylinder parts i'm finding just as low priced...
    And there are no nicks or tears in the seals, im not so confident new seals would improve things, these brakes have always been weak, perhaps my bore was a bit wide...

    i did find what seems an exact new part though...
    http://www.ebay.com/itm/181969475724
    i may well go this route short of a definitive rebuild kit part number, and hopefully reuse my modified adjustable levers without having to do all that work again.
    Last edited by axa; 05-31-2017 at 03:37 PM.
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  4. #6034 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru axa's Avatar
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    Quote Originally Posted by barnone View Post
    6-54.jpg

    Could there be a problem with the master cylinder check valve which I assume the DF has? Maybe a bit of something that does not allow the valve to seal?
    I would bleed/flush the fluid first to see if that would cure the problem. If that didn't work then disassemble the master cylinder to look at the check valve.
    Snap, taking a close look at this diagram, when i took the cylinder off and up to the bench, i remember seeing the primary cup, secondary cup, but no check valve?!
    also rebuild kits don't seem to come with one so i didnt notice anything missing...
    I cant imagine they even worked as poorly as they did before without a check valve...

    thanks for the pic, I'll investigate, but like i mention none of the similar rebuild kits even list a check valve, are you guys sure there is even one?
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  5. #6035 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru axa's Avatar
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    Quote Originally Posted by ben2go View Post
    The check valve holds about 2psi in the system to keep the pistons from returning when the lever is released but it's not enough pressure to cause brake drag. Over on chinariders, there was a member that had a similar problem. He removed the master cylinder and did an old fashion bench bleed. During the bleed, some hardened chunky brake fluid debris came out of the hole that allows fluid to travel down through to the piston. You should be able to disassemble and clean the master cylinder without dmaging any seals. I have done it on my Jap bikes with no problems.

    This video matches and displays what parts are found in the DF master cylinder pretty well...
    there is no check valve, im not sure what you're talking about...
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  6. #6036 Re: Dong Fang DF 250 RTB Bobber Information 
    C-Moto Guru ben2go's Avatar
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    Some don't have check valves. Usually, the master cylinder being above the caliper will do the trick. Also, the reservoir getting low can cause a bit of suction pulling the caliper in. The rubber seal doesn't let air into the system because brake fluid is hygroscopic and absorbs water from the air. As the fluid level drops, if the bellows in the seal doesn't drop as well, there will be a vacuum created in the master cylinder, which will pull the caliper piston back. This is why the master cylinder cap seal is made the way it is.
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  7. #6037 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru axa's Avatar
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    Quote Originally Posted by ben2go View Post
    Some don't have check valves. Usually, the master cylinder being above the caliper will do the trick. Also, the reservoir getting low can cause a bit of suction pulling the caliper in. The rubber seal doesn't let air into the system because brake fluid is hygroscopic and absorbs water from the air. As the fluid level drops, if the bellows in the seal doesn't drop as well, there will be a vacuum created in the master cylinder, which will pull the caliper piston back. This is why the master cylinder cap seal is made the way it is.
    some dont have it eh... plenty of info on the web about other bikes wouldnt you say.... you had me running around for an hour looking where i could have dropped the seal
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  8. #6038 Re: Dong Fang DF 250 RTB Bobber Information 
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    Quote Originally Posted by axa View Post
    some dont have it eh... plenty of info on the web about other bikes wouldnt you say.... you had me running around for an hour looking where i could have dropped the seal
    Sorry bout that. I should have known the china bikes wouldn't have them as a cost cutting measure.
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  9. #6039 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru barnone's Avatar
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    Quote Originally Posted by axa View Post
    some dont have it eh... plenty of info on the web about other bikes wouldnt you say.... you had me running around for an hour looking where i could have dropped the seal
    axa, we were trying to help you.
    Vince
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  10. #6040 Re: Dong Fang DF 250 RTB Bobber Information 
    Senior C-Moto Guru barnone's Avatar
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    Quote Originally Posted by ben2go View Post
    Sorry bout that. I should have known the china bikes wouldn't have them as a cost cutting measure.
    That is the thanks you get for trying to help axa!
    Vince
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