Master cylinder replacement

[asg1986]

If you open the reservoir cap, then the bleed screws the gravity method is fine (unless your car is on its roof). If you open a bleed screw and then the reservoir cap you will get air bubbles in the lines. Gravity bleeding is a great way to renew caliper fluid as long as the level in the master cylinder doesn't drop too low (this would allow air into the lines).

[rgk]

Turn a bottle of water upside down and open the cap. What happens?

Due to atmospheric pressure, the fluid runs out and air rushes in to the bottle.

Fluid runs out, air rushes in. Fact and simple physics.

No air in brakes lines is acceptable. That's because brake fluid is a hydraulic fluid that does not compress well under pressure, which makes it ideal to transfer power from your foot to the brake pedal, to the brake pads, slowing and stopping the wheel's movement.

Air, as you might guess, is the opposite. It compresses very well under pressure. This makes it dangerous to have in your brake lines. When you have air in your brake lines, the pedal feels spongy - that's the feeling of squeezing air. The result is less (if any) power transferred to the brake pads.

[asg1986]

Here's a great analogy: when you shotgun a beer there isn't any air getting in your mouth, just cold beer . If you chug a beer without poking a hole in the bottom it goes down a lot slower (because air needs to fill the void in the can). That's how this works. Beer (or brake fluid) is heavier than air. As long as you have atmospheric pressure pushing down the brake fluid from the reservoir you won't have air entering the bleeder. If you have the bleeder open and not the reservoir cap you will introduce air into the lines (chugging). Do a google search on gravity bleeding - trust me it works.