Master Cylinder / Brake Pedal Relationship

MASTER CYLINDER / BRAKE PEDAL RELATIONSHIP





WARNING: Master Cylinders can be easily misdiagnosed for problems related to excessive brake pedal travel.

Excessive brake pedal travel is often associated with a defective master-cylinder, however total pedal travel is affected by a wide variety of components and issues.




Issues Affecting Pedal Travel

Brake Fluid
- Brake fluid which is contaminated with air or water will significantly add to the amount of pedal travel.
- Pure DOT 3 (or 4) brake fluid is incompressible (allowing for a solid, firm pedal). A small amount of air trapped in the brake fluid will require extra effort from the master-cylinder to compress it, resulting in a soft and spongy pedal.
- During prolonged or severe braking the brake fluid temperature can rapidly rise above 212 degrees F. When this occurs any water in the brake fluid will boil into steam (which is compressable) and create a soft and spongy pedal.

Rear Brake Shoe Adjustment
- Drum brake system utilize return springs to pull the shoes away from the drums when not in use. The amount of distance the shoes have to extend to meet the drums greatly affects the amount of pedal travel.
- Shoes/Linings which are badly out of adjustment can by themselves result in a brake pedal sinking all the way to the floor

NOTE: Improperly adjusted rear shoes/linings also affect the parking brake.

Calipers
- Excessive rotor wobble caused by a warped rotor or loose/worn wheel bearings can knock the caliper piston further inward from its normal resting position. This results in additional pedal travel required to extend the piston and apply the brakes.

Drum Expansion
- Drums which are worn past their "Discard" thickness are prone to expanding outwards into an oval shape during heavy braking. This drum expansion results in additional brake pedal travel.




Brake Fade
- During prolonged or severe braking, the amount of pedal effort/travel required to slow the vehicle increases as the ability of the brakes to dissipate heat decreases.
- As the brake linings heat up, their "coefficient of friction" is reduced (they become slicker). As the coefficient of friction is reduced, more hydraulic pressure is required to stop the vehicle. More hydraulic pressure results in more heat which then results in more pedal fade.
- As the brake linings, rotors, and drums begin to wear, their ability to absorb and release heat is reduced significantly. This makes worn brakes more prone to "pedal fade".

NOTE: Prior to replacing a master-cylinder, verify the entire brake system is functioning properly.