Air trapped in brake lines due to leaks or the replacement of parts will make the pedal feel spongy and reduce the force with which the linings are pressed against the drums or discs. This air must be forced out through bleed valves at each wheel.
If your car has power brakes, begin by pumping the brake pedal ten times, with the engine off, to use up any vacuum in the booster. Raise the car and attach a length of clean vacuum hose to one of the bleed valves. Insert the other end of the hose into a jar partly filled with fresh brake fluid. Use a wrench to open the valve three-quarters of a turn. Have a helper apply the brakes slowly but steadily.
When the pedal reaches the floor, close the valve; then have your helper let the pedal slowly return to its normal position. Repeat this procedure until air bubbles stop coming out of the hose. Keep adding fresh brake fluid to the reservoir so that it will not run dry. To replace all the fluid in the system, bleed each wheel until the old fluid runs clear.
On some disc/drum systems, a metering valve found under the master cylinder may prevent fluid from being pumped out of the front disc brakes. In that case, you must hold the metering valve open.
When you have finished bleeding the brakes, the brake warning light on the dash will be lit. To turn it out on the late model cars, turn the ignition switch to the Accessory or On position and apply the brakes once.
Replace warning light switch on older cars with special stud. Ask a parts dealer if your car needs this part. Remove the dust cap (if any) and clean off the bleed valve. It is on the backing plate of drum brakes. Bleed valve is found on calipers of disc brakes. Put box wrench over valve, then force on hose. Have your helper pump brakes until the bubbles stop or darkened fluid is displaced, keep hose submerged.
On older model cars, you must center the switch inside the metering valve housing to shut off the light. To do this, open a bleed valve in the circuit opposite the one that was bled last. Have your helper press the brake pedal slowly just until the light goes out. Close the bleed valve immediately. If the light goes on again, you have overshot the center of the switch and must repeat the procedure at the opposite end of the car. You can avoid this tedious procedure by replacing the light switch with a small threaded stud-sold in auto part stores-before bleeding the brakes. If you have a 1974 or earlier American Motors car, unscrew the brake warning light switch from the metering valve before you begin. Replace it when you are through.
Before driving the car, start the engine and pump the brakes to test for a firm pedal. If the pedal still feels spongy, there is still air in the system. The following cars must be power bled by a mechanic:Before driving the car
All Chrysler Corporation cars from 1969 onward, 1969 Lincolns, and 1970-72 Fords, Lincolns, and Mercurys.