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A flexible brake hose whose inner liner has delaminated, letting pressure in and not out. The outside looks perfect, which is what makes this one of the most missed faults in braking — the hose passes every visual inspection and behaves as a valve.

The caliper applies normally and then will not release, so that wheel drags, overheats and wears its pads and disc rapidly, and the vehicle pulls. The distinguishing test is definitive: with the wheel dragging, crack the bleed nipple — if fluid spurts and the wheel frees, pressure was trapped, and the hose is the cause rather than the caliper. That distinguishes it from a seized caliper piston, which produces identical symptoms and a completely different repair. Replace hoses in axle pairs, since they are the same age.