Given the mileage you listed, a defective master cylinder is not uncommon. Also, many people damage a master cylinder by bleeding improperly with a helper or when surprised by a sudden leak. In either case, if you push the brake pedal all the way to the floor, there is about an eighty percent chance the master cylinder will be damaged. Crud and corrosion build up in the lower halves of the bores where the pistons do not normally travel. Pushing the brake pedal more than half way to the floor runs the rubber lip seals over that crud and can rip them. Most commonly that causes a slowly-sinking brake pedal, and that often does not show up until two or three days later. Professionals avoid this by never pushing the brake pedal over half way, and very few of us use a helper to pedal-bleed the brakes. I have only used gravity-bleeding for the last twenty five years, partly because I am by myself when working on my own old rusty stuff. Pushing the brake pedal to the floor is not a concern when the master cylinder is less than about a year old, but it is still a good practice to not push the pedal over half way.
Regardless of the cause, if we are to assume your old master cylinder was the cause of a low brake pedal, you should not even consider installing the replacement until it has been bench-bled. All replacement units come with the hoses and the correct plastic fittings for the job.
Next, there is a trick that allows you to not have to bleed at the wheels. When you replace the master cylinder with two steel lines, loosen the line nuts a little, remove the mounting bolts to the power booster, pull the master cylinder forward, then use it as a handle to bend the steel lines up a little. That will keep the fluid from running out of the lines.
Remove the two lines all the way, then remove the master cylinder. Brake fluid eats paint, so be careful to not allow any to drip onto the car.
Screw the two lines into the new master cylinder that has been bench-bled, then use it to bend those lines back down to their normal shape. Bolt it to the booster, then snug one of the line nuts. Have a helper slowly push the brake pedal half way to the floor. It should take about fifteen seconds to do that. You will see bubbles coming out by that nut. Snug the nut, then holler to the helper to quickly release the pedal.
Loosen the nut, then do that a second time, and perhaps a third time, until you see only clear fluid with no bubbles coming out, then do that for the other line. By pushing slowly, fluid will get pushed down the lines, and air will float back up. By releasing the pedal quickly, the fluid rushing back will wash the air back up into the reservoir with it. This can even work when working on the car by yourself, just keep the line nuts tight. If the bleeder screws by the wheels are never opened, there is no way air up by the master cylinder can get down to the wheels or get trapped in the ABS hydraulic controller.
If you do try to bleed at the wheels, the air in the lines by the new master cylinder is going to become trapped in those two chambers in the ABS hydraulic controller. You will not get that air out without a scanner.
Monday, April 30th, 2018 AT 3:11 PM