Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the

brake fluid Brake fluid is a type of hydraulic fluid used in hydraulic brake and hydraulic clutch applications in automobiles, motorcycles, light trucks, and some bicycles. It is used to transfer force into pressure, and to amplify braking force. It works ...

) are purged of any air bubbles. This is necessary because, while the brake fluid is an

incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow ( isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An eq ...

liquid, air bubbles are

compressible In thermodynamics and fluid mechanics, the compressibility (also known as the coefficient of compressibility or, if the temperature is held constant, the isothermal compressibility) is a measure of the instantaneous relative volume change of a f ...

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system. The same methods used for bleeding are also used for brake flushing or purging, where the old fluid is replaced with new fluid, which is necessary maintenance.

Methods

The process is performed by forcing clean, bubble-free brake fluid through the entire system, usually from the

master cylinder In automotive engineering, the master cylinder is a control device that converts force (commonly from a driver's foot) into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic brake system. A ...

(s) to the calipers of disc brakes (or the

wheel cylinder A wheel cylinder is a component of a hydraulic drum brake system. It is located in each wheel and is usually positioned at the top of the wheel, above the shoes. Its function is to exert force onto the shoes so as to bring them into contact wit ...

s of drum brakes), but in certain cases in the opposite direction. A brake bleed screw is normally mounted at the highest point on each cylinder or caliper. There are five main methods of bleeding:Brake Bleeding Methods
by Phoenix Systems *The ''pump and hold method'', the brake pedal is pressed while one bleed screw at a time is opened, allowing air to escape. The bleed screw must be closed before releasing the pedal, or a

one-way valve A check valve, non-return valve, reflux valve, retention valve, foot valve, or one-way valve is a valve that normally allows fluid ( liquid or gas) to flow through it in only one direction. Check valves are two-port valves, meaning they have ...

must be fitted. *In the ''vacuum method'', a

vacuum pump A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto ...

is attached to the bleeder valve, which is opened and fluid extracted with the pump until it runs clear of bubbles. *In the ''pressure method'', a pressure pump is attached to the master cylinder, pressurizing the system, and the bleeder valves are opened one at a time until the fluid is clear of air. Specialised pumps may incorporate a method to automatically keep the brake fluid reservoir full during bleeding. *In the ''reverse method'', a pump is used to force fluid through the bleeder valve to the master cylinder. This method uses the concept that air rises in liquid and naturally wants to escape up and out of the brake system. *''Gravity bleeding'' is a simple and easy method replace automotive brake fluid. It can also be used to bleed systems containing air bubbles but may not be as effective as other methods.

Pros and cons of different methods

The pump and hold method usually requires two people, although it can be performed by one person if one-way bleed nipples or valves are fitted. During this method, the brake pedal and master cylinder piston will travel beyond their normal range which can cause wear on seals or incorporate corroded metal into the brake fluid. Multiple wheel cylinders or calipers can be bled simultaneously, although this is only usually done with the gravity method which doesn't require pumps. The gravity method does take a long time, typically several hours to replace all the fluid. Bubbles in the system may rise faster than the fluid drains, so it may not be effective at bleeding, but is generally sufficient for replacing old fluid with new. Gravity bleeding is a very simple method requiring only one person. When the reservoir cap is removed and a bleed nipple opened, fluid drips slowly from the nipple because it is lower than the reservoir. A hose is typically used to collect the fluid in a container for safe disposal. The reservoir must be kept above the min mark at all times to avoid more air being drawn into the system. Applying

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

to the reservoir or bleed nipples, or applying a

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...

to the bleed nipples, makes the fluid flow through much more quickly than gravity alone. Replacing all the fluid takes only a few minutes. Different vehicles have different recommended bleeding patterns. Brakes are usually bled starting with the wheel that is furthest from the master cylinder and working towards the wheel closest to the master cylinder. This prevents bubbles in the system being forced into pipes towards wheels that have already been bled.

Bench bleeding

After replacement of the master cylinder, the master cylinder is usually "bench bled" before installation. Typically by securing it on the bench, filling it with fluid, connecting fittings and hoses to route fluid from the outlet ports on the master cylinder back to its reservoir, and repeatedly depressing the master cylinder plunger until bubbles are no longer seen coming from the hoses.

Clutch bleeding

The same techniques are used for bleeding hydraulic clutch systems. A bleed nipple on the clutch

slave cylinder In automotive engineering, the master cylinder is a control device that converts force (commonly from a driver's foot) into hydraulic pressure. This device controls slave cylinders located at the other end of the hydraulic brake system. As pi ...

is used. Gravity bleeding is often not effective because of the layout of the clutch pipework.

References

External links

Another "how to" on bleeding brakes.

How To Bleed Your Brakes By Yourself In Less Than 10 Minutes
{{Powertrain Vehicle braking technologies