The principle by which hydraulics work is that liquids are NOT compressable, while air is.
"Bleeding the brakes" simply means you bleed any air that may have entered the brake system (which is hydraulic). Whenever you remove a hydraulic component on the brakes you MUST bleed the air that has entered the system, or the brake pedal will go to floor or pedal will feel spongy due to air in system.
There are "bleed screws" at each caliper or wheel cylinder that allow bleeding.
The master cylinder is, of course, a major hydraulic component.
If a mechanic changed the MC and did not bleed the brakes, I would avoid him.
Thursday, March 26th, 2009 AT 7:15 AM