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I braked hard today and felt the pressure in my disk brake drop very suddenly.

Since that point the brake has not been working with no action at the calliper and little pressure in the lever but not spongy.

Any one know the likely cause and whether a bleed might fix.

I can't see any evidence of fluid leakage.

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Just like brakes on a car, mostly. Could be an adjustment that slipped, could be fluid leakage, could be a bad master cylinder. Have you checked that you have enough fluid in the reservoir? –  Daniel R Hicks Jun 24 '12 at 22:28
    
No reservoirs its a sealed circuit of Master to Slave. Its the odd thing. I can't find any fluid leakage but I haven't pulled the pads out yet to see if the pistons have blown seals. –  Warren Burton Jun 25 '12 at 14:25
    
There is a reservoir associated with the master cylinder. –  Daniel R Hicks Jun 25 '12 at 14:54
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2 Answers

I can't imagine how it did it as I didn't think there would be enough clearance between calliper and rotor, but the actual problem now I have gotten around to pulling off the back wheel is that the mobile pad got thrown and hence no fluid leak just a missing pad.

@zenbike thanks for answer however.

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Sounds like a major fluid loss, somewhere. Check around the hose joints, and the brake levers and calipers for fluid leakage. If not, it's probably a blown piston seal, which in rare cases can leak, but reseal under low fluid pressure.

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