Master Cylinder Flashcards

A vehicle's brake pedal slowly sinks toward the floor while steady pressure is held. There are no external leaks and the reservoir level has not dropped. What is the most likely cause?

Answer: Worn or damaged piston cups inside the master cylinder bore

A pedal that sinks with no external leak and no fluid loss points to fluid bypassing the master cylinder piston cups internally.

During a master cylinder replacement, the pushrod is adjusted too long so that it holds the primary piston forward with the pedal released. Which symptom is most likely?

Answer: Brakes that drag and overheat because the compensating port stays covered

A pushrod that is too long keeps the primary cup over the compensating port, so pressure cannot release and the brakes drag.

When bench bleeding a master cylinder, why are the bleed tubes or fittings routed back into the reservoir with their ends below the fluid level?

Answer: To prevent air from being drawn back into the bore on the piston's return stroke

Keeping the tube ends submerged means that only fluid, not air, is drawn back into the bore when the piston returns.

A technician finds brake fluid on the firewall and inside the vacuum booster behind the master cylinder, and the reservoir level keeps falling. What is the most likely cause?

Answer: A leaking rear (primary piston) seal in the master cylinder

Fluid at the booster and firewall points to the master cylinder's primary piston rear seal leaking out of the back of the bore.

A customer complains that the brake fluid level in the reservoir is slightly lower than at the last service. The vehicle has front disc brakes, and the pads are about half worn. A thorough inspection finds no leaks. What should the technician conclude?

Answer: The level drop is normal because fluid fills the caliper pistons as the pads wear

As disc pads wear, the caliper pistons extend farther and take up fluid, so the reservoir level falls gradually.

To test a vacuum brake booster, a technician pumps the pedal several times with the engine off, holds firm pressure, and starts the engine. What should happen if the booster and vacuum supply are working properly?

Answer: The pedal should drop slightly under steady pressure

When engine vacuum reaches the booster, the pedal drops a little as the booster provides assist.

A technician finds that a master cylinder's rubber cups are swollen and the fluid looks contaminated with petroleum-based oil. What is the correct repair?

Answer: Flush the system and replace the master cylinder, all rubber hoses and seals, and other contaminated components

Petroleum oil swells rubber seals and hoses throughout the hydraulic system, so all affected rubber parts must be replaced and the system flushed.

A brake pushrod gauge is used when installing a master cylinder on a power booster. What does the technician compare with this tool?

Answer: Booster pushrod protrusion against the depth of the master cylinder piston recess

The gauge checks that the booster pushrod length matches the master cylinder piston depth so the piston ends up with the correct clearance.

Why should brake fluid from a container that has been left open for a long time not be used when refilling the reservoir?

Answer: It absorbs moisture, which lowers its boiling point and promotes corrosion

Glycol-based brake fluid is hygroscopic, so open containers take on moisture that lowers the boiling point and causes corrosion.

All four wheels drag after the pedal is released. When the technician loosens the master cylinder outlet lines, the pressure releases and the wheels spin freely. Which is the most likely cause?

Answer: A plugged compensating port or a swollen cup in the master cylinder

If dragging stops when the lines are loosened at the master cylinder, pressure is being trapped upstream, so the master cylinder or its pushrod is at fault.