The master cylinder is a hydraulic control device that converts physical pressure (commonly from a driver's foot) into hydraulic pressure to operate other device(s) in the hydraulic system. The most common automotive uses of master cylinders are in brake and clutch systems. The operated device in the clutch system is called the slave cylinder. In brake systems, the operated devices are brake calipers and/or wheel cylinders. For other uses, see Clutch (disambiguation). ...
How a master cylinder works
A master cylinder is made up of two reservoirs of fluid, two pistons, and two or more circuits (brake lines). When the brake pedal is depressed, the primary piston moves in to create pressure in the reservoirs. This pressure, in conjunction with the secondary piston, compresses the fluid evenly through each circuit and into the brake calipers. Typically, engine vacuum is used to assist in applying pressure to the system. In case of failure of a brake line, the master cylinder will redirect hydraulic pressure to the remaining lines, thereby avoiding catastrophic failure of the brakes.
Reference
Nice, Karim: How Master Cylinders and Combination Valves Work, How Stuff Works.
The pressure is NOT created in the reservoir, but in the brake lines leading to the wheel cylinders.
If the mastercylinder is not bled, it will take you at least twice as long to bleed the system, and then there's no guarantee that you'll have removed all the air from the system.
Mastercylinders MUST be bench bled outside the vehicle and without any other components attached.
Remove the mastercylinder top and fill with fresh brake fluid to the appropriate level as indicated in the mastercylinder-- generally about 1/2 to 1 inch from the top of the reservoir.
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