Brake Bleeder Screw
A threaded valve on a brake caliper or wheel cylinder that is opened to purge air bubbles and contaminated fluid from the hydraulic brake system.
A brake bleeder screw (or bleeder valve) is a small, hollow metal bolt located on the top of each brake caliper (for disc brakes) or wheel cylinder (for drum brakes). It is the dedicated access port used to perform a brake bleed or fluid flush.
The bleeder screw has a tapered tip that seals against a matching seat inside the caliper. When closed, it prevents brake fluid from escaping under high pressure. When loosened slightly, it opens a passage allowing fluid and trapped air to flow out through the center of the screw. A rubber cap covers the tip of the screw to keep dirt and moisture out of the port.
Because bleeder screws are located in areas exposed to water, road salt, and extreme heat, they frequently rust and seize in place. Attempting to loosen a seized bleeder screw with too much force can cause it to snap off inside the caliper, requiring caliper replacement or extraction. Mechanics often use penetrating oil, heat, or specialized sockets to loosen stubborn screws.
Ensuring that the bleeder screws are in good condition and clear of debris is essential for proper brake system maintenance. During any brake service, checking for fluid leaks around the bleeder screw is a critical safety check.