When any part of the hydraulic system has been disconnected for repair or replacement, air may get into the lines and cause spongy pedal action (because air can be compressed and brake fluid cannot). To correct this condition, it is necessary to bleed the hydraulic system so to be sure all air is purged.
Bleeding must start where the lines were disconnected. If lines were disconnected at the master cylinder, for example, bleeding must be done at that point before proceeding downstream.
When bleeding the brake system, bleed one brake cylinder at a time, beginning at the cylinder with the longest hydraulic line (farthest from the master cylinder) first. Failure to do so may result in more air being drawn into the lines.
If the existing system fluid seems dirty or if the vehicle has covered considerable mileage, it is recommended that the system be completely purged and refilled with fresh, clean fluid. The best way to start is to siphon the old fluid out of the master cylinder reservoir and fill it completely with fresh fluid.
Brake fluid tends to darken over time. This does not necessarily indicate contamination. Examine fluid closely for foreign matter.
The primary and secondary hydraulic brake systems are separate and are bled independently. During the bleeding operation, do not allow the reservoir to run dry. Keep the master cylinder reservoir filled with brake fluid. Never use brake fluid that has been drained from the hydraulic system, no matter how clean it seems.
- Clean all dirt from around the master cylinder fill cap, remove the cap and fill the master cylinder with brake fluid until the level is within 1 / 4 in. (6mm) of the top edge of the reservoir.
- Clean the bleeder screws at all 4 wheels. The bleeder screws are located on the back of the brake backing plate (drum brakes) and on the top of the brake calipers (disc brakes).
- Bleeder screws should be protected with rubber caps. If they are missing, the orifice may easily become clogged with road dirt. If the screw refuses to bleed when loosened, remove it and blow clear. Aftermarket caps are readily available.