Brake bleeding

Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressiblegas 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 purging, where the old fluid is replaced with new fluid, which is necessary maintenance.

The process is performed by forcing clean, bubble-free brake fluid through the entire system, usually from the master cylinder(s) to the calipers of disc brakes (or the wheel cylinders 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 four main methods of bleeding:[1]

  • 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.
  • In the vacuum method, a specialized vacuum pump 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 specialized 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.
  • 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.

Close-up of a disk brake bleed screw

Vacuum bleeding a disk brake caliper

Pressure bleeding a brake system

If bleeding brakes because of master cylinder replacement 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.

Different vehicles have different 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.

References

  1. Brake Bleeding Methods by Phoenix Systems
Electric friction brake An electric friction brake, often referred to as just electric brake or electric trailer brake is a brake controlled by an electric current and can be seen on medium duty trailers like caravans/RVs and consumer-grade car trailers. It is related to the electromagnetic track brake used in railways which also use electric current to directly control the brake force. Mechanical principle This describes the electrically controlled drum brake principles.   The brake is built with the brake shield (1) as a base that contains the mechanism. The brake shield is mounted on an axle/spindle using the holes in the centre. The brake shoes (3) are the items performing the braking by pressing outwards at the drum that covers all the innards. The brake shoes are held in place by reactor springs (2) and an adjuster (7) spring. There are also some minor clips not pictured to keep the brake shoes in place. Braking starts with applying a current proportional to the desired brake f...
Electric park brake Electric park brakes (EPB) are used on passenger vehicles to hold the vehicle stationary on grades and flat roads. This was accomplished traditionally using a manual parking brake. With electric park brakes, the driver activates the holding mechanism with a button and the brake pads are then electrically applied onto the rear brakes. This is accomplished by an electronic control unit (ECU) and an actuator mechanism. There are two mechanisms that are currently in production: 1. Cable puller systems and 2. Caliper integrated systems. EPB systems can be considered a subset of Brake-by-wire technology. First installed in the 2002 BMW 7 Series (E65), electric park brakes have since appeared in a number of vehicles. Electric park brake in the center console in a Volkswagen Touran Functionality Apart from performing the basic vehicle holding function required of park brakes, the EPB systems provide other functions like automatic release of the park brakes when the driver presses the...
Brake A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction. Background Most brakes commonly use friction between two surfaces pressed together to convert the kinetic energy of the moving object into heat, though other methods of energy conversion may be employed. For example, regenerative braking converts much of the energy to electrical energy, which may be stored for later use. Other methods convert kinetic energy into potential energy in such stored forms as pressurized air or pressurized oil. Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transform kinetic energy into different forms, for example by transferring the energy to a rotating flywheel. Brakes are generally applie...
Parking brake In road vehicles, the parking brake, also called hand brake, emergency brake, or e-brake, is used to keep the vehicle stationary and in many cases also perform an emergency stop. Parking brakes on older vehicles often consist of a cable connected to two wheel brakes at one end and the other end to a pulling mechanism which is operated with the driver's hand or foot. The mechanism may be a hand-operated lever, at floor level beside the driver, or a straight pull handle located near the steering column, or a (foot-operated) pedal located beside the drivers leg. In most automobiles the parking brake operates only on the rear wheels, which have reduced traction while braking. Some automobiles have the parking brake operate on the front wheels, for example most Citroens manufactured since the end of World War II, and the early models of the Saab 900. Hand brake lever from a Geo Storm. In this photo, the lever mechanism is shown not installed in the car. Brake warning light ISO symb...
Emergency brake assist Emergency brake assist (EBA) or brake assist (BA or BAS) is a generic term for an automobile braking technology that increases braking pressure in an emergency. The first application was developed jointly by Daimler-Benz and TRW/LucasVarity. Research conducted in 1992 at the Mercedes-Benz driving simulator in Berlin revealed that more than 90% of drivers fail to brake with enough force when faced with an emergency. By interpreting the speed and force with which the brake pedal is pushed, the system detects if the driver is trying to execute an emergency stop, and if the brake pedal is not fully applied, the system overrides and fully applies the brakes until the anti-lock braking system (ABS) takes over to stop the wheels locking up. This is a lower level of automation than a collision avoidance system, which may initiate braking on its own if the onboard computer detects an imminent collision. Overview Many drivers are not prepared for the relatively high efforts required for ...