Final answer:
A slow leak in the brake pipe would cause the brakes to "creep off" most quickly.
Step-by-step explanation:
When a car is laid up on a yard track with the air brakes applied and the air compressor cut out of service, the brake would "creep off" most quickly by a slow leak in the brake pipe. The brake pipe is part of the air brake system and a slow leak in this pipe would cause a decrease in air pressure, leading to the brakes releasing or creeping off.
The brake pipe connects the air compressor in the main reservoir to the brake cylinders of each wheel. The air pressure in the brake pipe is used to keep the brakes applied. If there is a slow leak in the brake pipe, the air pressure decreases, causing the brakes to release.