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When changing hydraulic components of the brake system, the air needs to be bled out of the brake lines. This also needs to be done when replacing the brake fluid per the periodic maintenance schedule. Also be sure to bleed the brake master cylinder if it runs dry.

1 Overview
2 Master Cylinder Bleeding
3 One-man Method
4 Method 2
5 Traditional Two-man Method
6 Other methods
7 Flushing

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Overview

The hydraulic system must be bled whenever any line has been disconnected or air has in some way entered the system. "Spongy" pedal action is an indication that air has entered the system -- if the brakes "pump up" (get stiffer after you press the pedal rapidly a few times) that is another tell-tale sign of air in the lines . If however the pedal is "soft" (has excessive travel), then adjust the drum brakes, then afterward re-evaluate the pedal action. If it's still spongy, proceed with bleeding.

Bleed the master cylinder (sometimes called "bench bleeding" but can be done while installed in the car)
Bleed the front wheel cylinders
Bleed the rear wheel cylinders

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Master Cylinder Bleeding

When replacing the master cylinder, bleed it before attaching to car.

Insert clean vinyl hose into front Outlet. Loop the hose into the reservoir
Insert clean vinyl hoses into REAR outlet. Loop the hose into the rear reservoir
Top up reservoirs with new brake fluid of recommended type
Work pushrod back and forth until air bubbles cease to appear in reservoir

If an installed master cylinder runs dry, bleed it as follows BEFORE bleeding individual wheel cylinders.

Top up reservoirs with new brake fluid of recommended type
Have assistant pump pedal three times, then hold pressure
Crack the fitting slightly for front outlet of master cylinder until fluid comes out
Tighten fitting
Repeat steps until only fluid comes out, and no air spurts out with the fluid.
Repeat for rear outlet port of master cylinder

If necessary, top up reservoir with new fluid. DO NOT REUSE BRAKE FLUID.

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One-man Method

The fluid prevents air from going back up into the cylinder, and can be done by one person (no assistant needed).

Put about 1" new fluid in a clear glass. Place glass on ground
put a hose from the bleeder to the bottom of the glass, making sure the new fluid covers end of hose
Crack the bleeder slightly
pump the brake pedal while you watch the glass (lie on the ground with the door open, and work the pedal with your hand). When air bubbles stop coming out, it's done. Often this only takes two or three pumps.
tighten bleeder
top-up the master cylinder. DO NOT REUSE BRAKE FLUID
bleed the next wheel cylinder

Any spilled brake fluid should be rinsed off painted surfaces with lots of water.

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Method 2

Do it this way and you'll do it only once and get it right the first time ... I had my brakes finished in about 25 minutes on my own compared to the hours and hours I spent doing it with a second person.

Discussion: Bleeding the brakes.....on your own

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Traditional Two-man Method

The traditional method of brake bleeding requires an assistant to work the pedal, while the mechanic tends to the wheel cylinders.

The Datsun 1200 Factory Service Manual recommends a modification of this by using a hose from the bleeder screw into a jar with a bit of brake fluid in it covering the hose end.

have assistant pump the brake pedal three times and then hold pressure
loosen bleeder (the pedal will travel down under assistant's foot pressure)
have assistant keep pushing on brake pedal (do not let up)
tighten bleeder
let brake pedal up
repeat until a clean, steady stream of brake fluid comes out. Often only two pedal cycles are needed
repeat for the other three wheels

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