If a brake pedal feels excessively low, what is likely a contributing factor?

When a brake pedal feels excessively low, a weak return spring can indeed be a contributing factor. The return spring is responsible for pulling the brake pedal back to its resting position after it has been pressed. If this spring is weak or damaged, it may not be able to return the pedal fully to the starting position. As a result, the pedal can remain in a lower position, giving the driver the sensation of a soft or excessively low pedal.

Considering the other options, high brake fluid pressure would typically result in a firmer brake pedal feel, as the pressure should help in creating a strong braking force. Worn brake lines may lead to brake fluid leaks or other hydraulic issues but would not directly cause a low pedal feel unless they were significantly compromised and affecting the hydraulic system. Over-tightened calipers could actually cause binding, which would likely result in a high pedal feel or difficulty in engaging the brakes rather than a low pedal issue. Thus, a weak return spring directly contributes to the problem of an excessively low brake pedal.