What is a likely cause for a brake pedal that moves slowly to the floor while maintaining pressure at a stoplight in a vehicle with power disc/drum brakes?

• A. Damaged brake lines
• B. Leaking primary piston cup in master cylinder
• C. Worn brake shoes
• D. Faulty brake booster

Answer: (B)

A brake pedal that moves slowly to the floor while maintaining pressure typically indicates an issue within the hydraulic system of the brake system. The choice indicating a leaking primary piston cup in the master cylinder is correct because when there is a leak in the primary piston cup, it can allow fluid to bypass the seals. This causes the pedal to gradually sink as the fluid pressure in the system decreases due to the loss of hydraulic force.

In this case, while the vehicle is stopped, and if the brake pedal is held down, the fluid can escape under pressure through the defective seal. This results in a sensation of the pedal traveling slowly to the floor, as there is insufficient pressure being maintained to keep the pedal firm. This condition can lead to significant brake performance issues; therefore, addressing the master cylinder's integrity is crucial for safe operation.

Other factors like damaged brake lines or worn brake shoes may contribute to different types of brake failure, such as fluid loss or reduced braking efficiency, but they would not typically cause the same slow pedal drop situation described here. A faulty brake booster primarily affects the assist given to the brake pedal during application, rather than the hydraulic pressure maintenance once the pedal is held down.