VEHICLE BRAKE SYSTEM

A vehicle includes wheels, brakes to control speeds of the wheels, and a hydraulic fluid source in fluid communication with the brakes. The vehicle includes a computer storing instructions to determine that speeds of the wheels are zero and actuate the hydraulic fluid source to provide hydraulic fluid to a first set of one or more of the brakes at a first pressure sufficient to maintain the vehicle body at a present position and to provide hydraulic fluid at a volumetric rate to one or more of the brakes not in the first set, and, while providing hydraulic fluid at the volumetric rate to a brake not in the first set, detect second pressures of the brake not in the first set. The instructions include instructions to determine a volume-pressure curve for the brake not in the first set based on the volumetric rate and the second pressures.