A5: Brakes

Pascal's law: pressure applied to a confined, incompressible fluid is transmitted equally in all directions. Because liquids do not compress, force on the master cylinder piston is transmitted undiminished to every wheel, and piston-area differences multiply force.

The master cylinder converts the mechanical force from the brake pedal (and booster) into hydraulic pressure, sending fluid to the wheel cylinders and calipers. Modern units are tandem (dual) designs serving two separate circuits.

A tandem master cylinder has two pistons feeding two independent hydraulic circuits. If one circuit fails (a leak), the other still provides braking, preventing total brake loss. This is a federal safety requirement on modern vehicles.

Circuits split either front/rear or diagonally (one front and the opposite rear wheel per circuit). Front-wheel-drive cars favor a diagonal split so that if one circuit fails, each remaining circuit still has one front brake, keeping braking balanced and reducing pull.

The compensating port connects the cylinder bore to the reservoir when the pedal is released. It allows fluid to return to the reservoir and lets fluid replenish the bore as pads wear, preventing pressure buildup from thermal expansion. A blocked compensating port can cause brakes to drag.

A blocked or covered compensating port (often from a swollen primary cup, incorrect pushrod adjustment, or no pedal free play) prevents fluid from returning to the reservoir. Heat expands the trapped fluid, applying the brakes progressively until they drag and overheat.