The Toyota U660 and U661 automatic transmission is engineered for durability and efficiency but may develop issues in the valve body's critical hydraulic circuits over time. Key areas of concern include main line pressure and main boost pressure, which are vital for maintaining consistent performance and overall system efficiency. Secondary pressure and lube pressure may degrade with wear, potentially leading to overheating or insufficient lubrication. Problems with torque converter apply and release pressure can result in slipping or shuddering during operation. Additionally, solenoid regulation pressure and accumulator pressure wear may lead to delayed responses or erratic shifts.