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:
- 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.
- Brake Bleeding Methods by Phoenix Systems