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The brake Master Cylinder

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This series is divided into seven sections:
Part 1 - Why it goes bad and what it feels like (and why your pedal goes to the floor)
Part 2 - Removal and replacement
Part 3 - How to bench-bleed the new Master Cylinder
Part 4 - What the Master Cylinder looks like inside
Part 5 - How the Master Cylinder works, and why the pushrod needs to be set correctly
Part 6 - How to check the pushrod freeplay (a link to the bottom of Part 2)
Part 7 - How to adjust the pushrod freeplay
Note: All pictures can be clicked on for LARGER versions!

PART 1: Why it goes bad and what it feels like

Many of us have experienced it at some time or other: You're sitting at a light, pressing softly on the pedal, just enough to hold the car still, and the pedal slowly begins descending, all the way to the floor. You think you're imagining it, but down it goes. Just to be sure, you let the pedal rise again and stomp on it, but now it stays up! What's going on?

If you get the symptoms above (very gentle, soft application slowly drops the pedal to the floor; hard application sees it remain high) then your master cylinder seals are worn out or damaged. If you had a line leak, or a leak at one of the wheels, the pedal would go to the floor at all times, however slowly. Pushing hard or soft would not change that.

Eventually, if you're dumb enough to drive with a pedal that goes to the floor, it will go to the floor all the time when you press the pedal, but you'd have to ignore the symptoms for a long time.

Line leaks and wheel cylinder leaks eventually result in obvious fluid on the backs of the tires, or on your driveway. And the reservoir level will drop. Air in the lines can also cause the pedal to go to the floor, by usually results in a spongy, soft pedal. Master cylinder leaks are invisible. No drips, no wet spots, no change in the reservoir level. All you have is the dropping, spongy pedal.

What to do about it? There's only one thing: Replace it. It's easier than you might think.

Three things will cause the master cylinder to leak:
1) Old age
2) Neglecting fluid changes
3) Using the pedal-pump method of fluid bleeding in a car that has not had the fluid changed regularly

1) Old age is simply the fact that the seals can't rub back and forth against a metal surface for too many years before the action abrades them to the point that they are unable to hug the cylinder walls tightly any more. I just had this happen to my Integra. :( It looks like 248,000 miles is about the limit.

2) Neglected fluid changes will dramatically shorten seal life, since the fluid absorbs water. The fluid in the reservoir drops as the brake pads wear, and is displaced by air that seeps in from the outside through the reservoir cap. Air carries moisture. Well, water is heavier than brake fluid and sinks to the bottom of the  master cylinder bore. And what does water do to aluminum? It makes a little black skunk-stripe of corrosion along the bottom of the bore. This corrosion is rough, and tears the seals up.

Even if your master cylinder's alloy is very resistant to corrosion, there is one more reason neglected fluid changes cause leakage: swollen seals. With the passage of many years the brake fluid absorbs water. The longer it's ignored, the higher the concentration of water present in the fluid. The seals are constructed of materials that valiantly resist absorbing water, but given quite enough time, eventually they do. That water swells the seals. Of course, a rubber seal can distort, but an aluminum master cylinder bore won't. And so the seals compress against the cylinder bore and distort. Permanently.
This is OK after a fashion, but then one day you decide to change the fluid. Well, now the water in the seals wants to migrate to the new fluid, resulting in the seals eventually SHRINKING as the water vacates its premises. The seals don't un-distort to fill their original space, they stay squished! So if you change your fluid without pumping the brake pedal and days or weeks later the pedal goes to the floor, guess what? The neglect caught up to you anyway.

Basically the difference between the two is this:
Change the fluid and there's an immediate leakage problem: torn seals
Change the fluid and days or weeks later there's a leakage problem: seal shrinkage.

3) This item is related to Item 2... In normal use, the master cylinder pushrod and seals only move about a half-inch or so every time you step on the pedal, even when you step hard. However, total travel is over an inch when the pedal goes to the floor when you do the pedal-pump bleed. This means that there is about a half-inch of master cylinder bore that never gets used except when the pedal goes to the floor. If fluid changes have not been done for five years or so, goop, gunk and corrosion builds up in this unused portion. When you then bleed with the pedal-pump method and the pedal goes to floor afterwards, you've damaged the rubber seals agains this buildup.