What could cause parking brakes to bleed off pressure apart from an external leak?

The phenomenon of parking brakes losing pressure can indeed be attributed to factors beyond just external leaks, and one significant cause is an internal leak in the master cylinder.

When there is an internal leak, the fluid that should be retained within the master cylinder can bypass seals or other components, leading to a gradual loss of pressure necessary to hold the brake in place. This internal malfunction might not be immediately apparent, as there may not be any visible signs of leakage, unlike external leaks that often result in fluid pooling under the vehicle.

In contrast, a faulty brake pedal generally refers to issues like mechanical failures or improper alignment, which do not typically lead to a loss of hydraulic pressure independently. A broken cable specifically pertains to mechanical systems rather than hydraulic systems, while an empty reservoir would likely cause the brakes to fail entirely rather than simply bleed off pressure over time.

Thus, the internal leak in the master cylinder effectively explains how parking brakes could lose pressure without any external evidence of failure, showcasing the subtle complexities of brake system maintenance and functionality.