This is the number one thing customers do wrong when trying to do their own brake work. If the car is more than a year old, you must NEVER push the brake pedal to the floor.
You had to push the pistons into the calipers to make room for the new thicker pads. When the work is finished, you must pump the brake pedal a number of times to push the pistons out until the pads contact the rotors. What every professional knows is there is a huge buildup of crud in the lower half of each bore in the master cylinder where the seals don't normally travel. By allowing the pedal to go down too far, both lip seals were ripped and are now leaking internally.
You will need to replace the master cylinder. There are a couple of tricks to make this a better repair. First, crack the two line nuts so they'll be loose. Loosen one of the caps on the reservoir, then pry both front pistons into their calipers again. This will wash any debris that got pushed into the lines back into the master cylinder. Unbolt the master cylinder from the power booster, then use it as a lever to bend the two steel lines upward just a little. This will keep the brake fluid in the lines rather than letting it run out.
Be sure to follow the instructions for bench bleeding the new master cylinder. Attach the steel lines, and be sure to wash off any brake fluid that spills on painted surfaces. Bend the lines back down and bolt the master cylinder to the booster. Leave the line nuts just finger tight for now.
Since this a new master cylinder, it's ok to push the brake pedal down further than normal but it's still a good idea to get in the habit of avoiding going all the way to the floor. Have a helper push the brake pedal down SLOWLY. It should take about 20 seconds to go half way to the floor. While this is happening, loosen one steel line nut and you'll see air bubbles coming out there. Your helper must not allow the pedal to move back up until you have the line nut tight or air will be drawn in. Repeat this for the second line. It's important to move the pedal down slowly, otherwise air will just get pushed down the lines and be harder to remove. You might have to do this a second time to be sure all the air is out.
After the lines are tight and you have a good solid pedal, push it down slowly, hold it for a few seconds, then allow the pedal to return quickly. This will wash any trapped air bubbles back into the reservoir. The same thing will happen from normal driving too but it will take longer. This procedure eliminates the need to bleed at the wheels and risk breaking a rusted bleeder screw.
The next time you replace front pads, don't push the pedal down more than half way. The master cylinder can also be damaged this way when a hydraulic line springs a leak. For this reason, many shops will quote repairs that include a new master cylinder in case it got damaged. There's no easy way to tell until the leak has been fixed.
Sunday, September 6th, 2009 AT 11:25 PM