Pressure Retention

Fig 1: Pressure Retention & Pressure Reduction Valve Positions - Pressure Retention

Courtesy of MAZDA MOTORS CORP.

When pressure is being retained, current flows to the pressure retention valve coil. The magnetic force generated in the coil by the current enables the armature to overcome the force of the valve spring.
As a result, the armature moves and closes the pressure retention valve.
When the pressure retention valve closes, the passage between the master cylinder and the brake caliper is blocked. This maintains the pressure of the brake fluid acting on the brake calipers.