The pressure differential valve is used to turn on the brake warning light and shut off the part of the hydraulic system that has a leak.

The pressure differential valve, also known as a brake proportioning valve, detects differences in pressure between two sides of the brake system. If there's a significant difference, indicating a leak or failure in one part, it activates the brake warning light and can isolate the faulty part to maintain braking capability in the rest of the system.