could be a sticking caliper, but sometimes the flexible brake hose can collapse internally & act like a one way valve, one way to find out if it's this, is to jack up the car (wheel may need to be removed) press brake pedal so that the brake sticks on & then undo the bleed screw, if undoing the bleed screw releases the brake, then the pipe is most likely the problem.
jim
January 2010
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