The bleed nipple, also known as a bleed valve, is a hollowed-out fastener that taps into the hydraulic fluid circuit. When the valve is loosened, it allows brake fluid to pass through its hollow body which is typically captured by attaching a hose pipe and catch container to the bleed valve tip.
A bleed valve is a critical component in removing trapped air from a hydraulic system, the procedure is known as bleeding the brakes. The bleed valve is also used when flushing the brake fluid.
Where is Bleed Valve Located?
A bleed valve/nipple is located on the uppermost part of the brake caliper body, and the caliper is located behind each road wheel. Every brake caliper must have a bleed valve located at the uppermost part of the caliper body, as that’s where trapped air naturally gathers.
Vehicles with rear drum brakes at the rear won’t use a brake caliper but will still employ a bleed valve on the brake cylinder which is accessed at the rear of the rear wheel on the heat shield.
Common Bleed Valve Issues
Typical brake bleed valve issues include the following:
Stripped, or Rounded Bleed Valve fastener head – Using an ill-fitting wrench commonly causes the fastener head to strip or become rounded, which causes the wrench to slip.
It is still possible to remove a rounded valve using a Vice grip, although the valve should ideally be replaced.
Sheared Bleed Valve – The valve breaks off at the neck when opened, leaving the remnant stuck in the caliper body. Removal is possible but will require special tools; often, it’s best to replace the caliper completely; a new caliper comes complete with a new valve.
Bleed Valve Corrosion – The valve base is tapered and, when tightened into the concave seat of the caliper, creates a fluid seal. However, in older brake systems, corrosion may form on the bleed valve base or the caliper seat, preventing a perfect seal.
Blocked Bleed Valve – The valve body is, as you know, hollow, and to prevent dirt and road grime from entering the cavity, the valve is fitted with a rubber dust cover cap. The problem is the caps become detached, and the valves clog up with much and often corrode out. Bleeding with such a valve isn’t possible and will need to be replaced, as will the dust cap.
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.