Brake Bleeder Screws

Brake bleeder screws (also called bleed nipples or bleed valves) are small hollow screws threaded into the highest point of each brake caliper or wheel cylinder, providing the access point for purging air from the hydraulic brake circuit. Each caliper has one bleeder screw; most rear drum wheel cylinders also have one. The screw has a hollow centre bore and a ground conical seat that seals the bore when tightened. A rubber or vinyl bleed hose connects to the hex body and is submerged in a container of clean brake fluid during bleeding to prevent air re-entry. Bleeder screws are made of steel and are vulnerable to corrosion, which is why they should be loosened at every brake service even if bleeding is not required — a seized bleeder screw at a future service can make complete brake bleeding impossible without caliper replacement.

Air in the brake hydraulic circuit is compressible, unlike brake fluid. Even a small air bubble in a caliper bore allows the brake pedal to travel further before building pressure — the pedal compresses the air bubble rather than immediately displacing fluid to the piston. In severe cases, air in the circuit results in a pedal that sinks to the floor with no braking effect. Bleeder screws provide the exit point for air and old fluid during the bleeding process. When the bleeder screw is slightly loosened, hydraulic pressure from the master cylinder (created by pressing the brake pedal or by a pressure bleeder) pushes fluid — and any entrained air bubbles — out through the hollow bore and down the bleed hose into the catch container. The screw is tightened before releasing pedal pressure or releasing pressure from the bleeder to prevent air from being drawn back in.

Corroded and seized screws: The primary problem. Steel screws in aluminium calipers are subject to galvanic corrosion; steel screws in cast iron calipers corrode from road salt and moisture. A seized screw cannot be loosened for bleeding without risk of twisting off the screw body inside the caliper — requiring caliper replacement or extraction. Prevention: loosen and re-tighten every bleeder screw at every brake service, and apply copper grease or anti-seize to the threads after cleaning.

Rounded hex body: Bleeder screws have small hex bodies (typically 8–10mm) and are easily rounded by incorrect-size or worn spanners. Use a dedicated brake bleeder spanner or box spanner — never an adjustable wrench. A rounded bleeder screw is very difficult to remove without specialist extraction tools.

Leaking screw seat: The conical ground seat can be damaged by overtightening, causing fluid to seep past the seat even when the screw is fully tightened. Replace immediately — even a slow weep depletes the brake system.

Level 1 — Preventive Loosening: At every brake service, attempt to loosen each bleeder screw by a quarter turn and retighten. If a screw won't move, apply penetrating oil and leave it to soak before retrying. Never force a screw that won't move with a penetrating oil treatment — apply heat from a heat gun (not a flame near brake components) to the caliper body around the screw, which expands the aluminium slightly and often frees the steel screw.

Level 2 — Bleeding Procedure: Bleed the brake system from the wheel furthest from the master cylinder to the closest: typically right rear, left rear, right front, left front. Use clean, compatible brake fluid. Never allow the master cylinder reservoir to run dry during bleeding — this introduces air at the master cylinder end and requires significantly more bleeding effort to clear.

Level 3 — Seized Screw Extraction: A snapped bleeder screw inside a caliper can sometimes be removed with a left-hand drill bit and screw extractor. If the screw snaps below the surface, caliper replacement is typically more cost-effective than machining. Apply anti-seize compound to all new bleeder screw threads before installation to prevent recurrence.