During a road test, if the brake pedal slowly goes to the floor with normal pressure, what is the likely cause?

The scenario where the brake pedal slowly goes to the floor with normal pressure typically indicates a problem within the hydraulic braking system. A likely cause in this case is a leaking master cylinder primary piston seal.

When the primary seal in the master cylinder is compromised, it allows brake fluid to bypass the piston rather than maintaining the hydraulic pressure needed to keep the brakes engaged. As a result, the pedal feels normal when first pressed, but the loss of pressure from the hydraulic failure causes the pedal to gradually sink to the floor. This is a key symptom of a failing master cylinder and indicates that the brakes may become ineffective, posing a serious safety risk.

This condition contrasts with other options: worn brake pads would not typically cause the brake pedal to sink slowly but may lead to decreased stopping power or noise when braking. Low brake fluid level might lead to the brake pedal feeling spongy but wouldn't generally sink all the way to the floor under normal pressure. A faulty brake booster would typically manifest with a hard brake pedal and would not necessarily lead to a pedal sinking slowly to the floor.

Thus, a leaking master cylinder primary piston seal effectively explains the gradual sinking of the brake pedal, making it the most appropriate answer.