The conditions of operation and functions of automatic transmission fluid (ATF) in the automatic transmission (AT) have been presented. It has been shown that the ATF quality has a significant impact on the correct functioning of an electrohydraulic AT controller. The phenomenon of ATF flow through gaps (clearance) in precision hydraulic pairs, referred to as “internal leakage” and characterized by “internal leakage rate”, has been analysed. It has been shown that excessive “internal leakage”, caused by excessive wear of hydraulic parts and low oil viscosity, may result in a fluid pressure drop in the AT hydraulic system and in incorrect functioning of the transmission unit. A model of the rate of internal leakage of hydraulic fluid through precision pairs in distribution valves has been proposed. In the model, an equation has been used that defines the rate of internal leakage in a precision hydraulic pair with a concentric annular gap. The model assumptions adopted and the test conditions have been specified. The values of clearance between components of the precision pairs in the electrohydraulic controller were determined by measurements carried out on a real controller that had been normally operated for quite a long time. The related outer diameters of pistons and spools and inner diameters of spool valve sleeves and hydraulic dampers were measured. Multi-variant tests were carried out on the model, with determining the total leakage values for the AT hydraulic system controller with different values of the degree of wear of the precision pairs and at different values of fluid temperature (viscosity and density) for the conditions of vehicle drive in gears “1” and “2”.