Brake bleeding is the process carried on hydraulic brake systems whereby the brake lines carrying brake fluid are purged of any air bubbles. This process is important, as brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system resulting in failure of brakes.
The process is performed by forcing clean, bubble-free brake fluid through the brake lines (through the entire system) usually from the master cylinder to the calipers of disc brakes. A brake bleed screw is mounted at the highest point on each caliper.
Brakes should be bled in a specific order. If you are bleeding a rear-wheel drive vehicle, start with the right rear wheel, then move to the left rear wheel; then do the right front wheel and finally the left front wheel.
We generally use ‘the pump and hold’ method. This method is performed via these steps:-
Step 1: Fill the reservoir with fresh fluid before starting, and be sure to refill the reservoir throughout the bleeding process, then attach the clear tubing over the bleeder screw and put the other end in a bottle to catch the fluid.
Step 2: Ask your partner to pump the brake pedal, and then hold the pedal down as far as it will go. Your partner must hold it there until other say it’s time to release it.
Step 3: Once your partner is holding the brake pedal in the completely pressed down position, turn the bleeder valve 1/4 turn. This will release the brake fluid and air. Step 4: After all the brakes have been bled, test the brake pedal for firmness.