Brake Bleeder Valve
A threaded screw valve on a brake caliper or wheel cylinder that is opened to release air and old fluid from the hydraulic lines.
A brake bleeder valve (or bleeder screw) is a small, hollow threaded bolt located at the highest point of each brake caliper and wheel cylinder. It features a tapered seat at its tip that seals the hydraulic passage when tightened, and a small nipple hole on its head where a drain hose can be attached.
When performing brake service or replacing fluid, the bleeder screw is loosened slightly (typically a half-turn) using a line wrench. This opens a channel allowing pressurized brake fluid and trapped air bubbles to escape. Because air compresses under pressure, any air in the lines causes a soft, spongy brake pedal and reduced stopping force; opening the bleeder allows the air to be purged from the system.
Because they are exposed to road water, salt, and extreme heat, bleeder screws are highly susceptible to rusting and seizing in place. Attempting to force a seized bleeder screw with a standard wrench often snaps the head off, requiring caliper replacement or extracting the broken threads. Applying penetrating oil, using heat, or using a specialized flare nut wrench prevents damage during service.