The hydraulic fluid is a glycol organic compounded which exhibits high temperature characteristics. However, it is amenable to moisture. Moisture content will lower boiling temperature of the hydraulic fluid. Because the high temperature from the brake shoe area can be conducted to wheel cylinders to raise the temperature of the hydraulic fluid, high percentage of moisture content in the hydraulic fluid may result in the boiling of the fluid. The hydraulic fluid will become compressible which will cause the failure of the brake system.In this research, an alarm device is developed which can detect moisture content in the hydraulic fluid by using the resistance measurement. When the percentage of moisture content exceeds the safe range, the device will send an alarm signal to the panel to warm the driver. It can be an inexpensive and easy-installed safety device to an automobile.