Dirt can block the balance valve in the parking brake oil circuit on one side of the excavator. When the excavator is moving, if pressure oil cannot pass through the small hole in the balance valve to enter the spring chamber and move the valve spool, the pressure oil will be unable to flow through the groove on the balance valve into the hydraulic system. This results in the parking brake force not being released, and the travel motor will not rotate, ultimately causing the excavator to veer off course.

Solution: The balance valve should be removed for cleaning, ensuring that no dirt enters during the disassembly and assembly process. Additionally, regularly replace hydraulic oil and filter elements that meet the specifications to maintain system cleanliness and smooth operation.