Overfilling the power steering fluid reservoir causes foaming of the hydraulic fluid, leading to premature wear of the system. Power steering fluid is hydraulic fluid that moves cylinders to reduce the force required to turn the wheels and provides lubrication. Foaming fluid provides less lubrication than normal, so the internal parts wear more quickly.
In the past, manufacturers used metal power steering fluid reservoirs that made overfilling common. The switch to translucent plastic reservoirs makes checking and filling the fluid easier. Many of these units are marked on the outside so the driver is able to check the fluid level with a glance instead of using a dipstick.
Take time to clean the reservoir before opening it to add more fluid. Contaminants in any hydraulic system cause problems with operation. Always use the fluid the vehicle manufacturer recommends for the car. Open the lid and slowly pour fluid to the maximum fill line. If the unit is overfilled, remove excess fluid, dropping the level to the maximum mark, to prevent further foaming.
If the power steering unit continues to lose fluid, it probably has a leak. Drivers may notice an oily spot on the driveway under the car when there is a leak. Leaks allow air in the system, which causes noise and affects steering. Unrepaired leaks lead to premature failure of the power steering unit.