Tucked into the top of every brake caliper is a small hollow bolt called the bleeder screw. It looks minor, but it is the single access point for getting air out of your brake hydraulics - and it is also one of the most commonly seized fasteners on the car. Here is what it does and how to handle it.
What the bleeder screw does
The bleeder (or bleed) screw is a tapered valve at the highest point of the caliper. Loosen it and it opens a passage that lets old fluid and trapped air escape as fresh fluid pushes through from the master cylinder. Because air rises, putting the valve at the top lets it capture bubbles that would otherwise soften the pedal. It is the tool that makes a proper brake bleed possible.
Why it seizes
The bleeder sits exposed to road spray, so its threads and the tiny opening corrode. Rust welds it into the caliper, and its soft hex head rounds off easily when you force it. A seized bleeder is the number-one reason a simple bleed job turns into a caliper replacement. Always hit it with penetrating oil first and use a proper six-point flare or bleeder wrench, never an open-end that slips.
Dealing with a stuck bleeder
Soak it in penetrant, tap it lightly to break the rust bond, and turn slowly with a snug six-point wrench. Gentle heat can help. If it rounds off or snaps, the caliper usually has to come off - see rebuild vs replace a caliper. Never leave a bleeder loose or missing; it must seal fully or air gets sucked back in and the pedal goes soft.
Keep it healthy
After bleeding, wipe the bleeder clean and fit the little rubber dust cap - that cap is what keeps the valve from rusting shut. A dab of anti-seize on the threads helps next time. If you are servicing calipers or pads, refresh the brake hardware and understand the whole caliper's job while you are in there.