You're driving along and notice your brake pedal seems soft. It almost feels like you're stepping on a sponge. Something is obviously wrong, and you need to find out the cause. Spongy brakes is always caused by air in the brake lines, but there can be several different causes for this.
Check the fluid level in the master cylinder. If the fluid level is low, you need to bleed your brakes. Fill the reservoir of the master cylinder until it's completely full.
Install a proper box-end wrench to the right rear wheel circuit bleeder valve. Maintain fluid level at half full or better in reservoir throughout the process.
Place a transparent hose over the bleeder valve. Submerge the end of the hose in a clear container partially filled with brake fluid.
Have your assistant depress the brake pedal fully and maintain steady pressure on it. Once he has steady pressure on it, loosen the bleeder valve to release air from the brake line.
Wait 15 seconds, then repeat steps 1 through 4 with the same tire until no more air bubbles come out.
Follow steps 1 through 5 for each additional brake. Fill the master cylinder reservoir completely after all the brakes are done. Check the feel of the brake pedal.
Repeat the bleeding process. This can expel any air still trapped in the system.
Check your brake lines for any leaks or tears. If you find one, replace the brake line and bleed your brakes again.
Call for a tow or bring your car to a mechanic if you cannot find a leak and you've bled your brakes at least twice. Something may be wrong with the master cylinder.
