Transmission hydraulic control unit

A hydraulic system (1) for an automatic transmission of a motor vehicle, the hydraulic system comprising primary and secondary pressure circuits (PK, SK) which can be loaded with hydraulic fluid from a pressure source (2) and in relation to a supply with hydraulic fluid of different areas of the transmission are differently prioritized, a pressure in each of the primary and the secondary pressure circuits (PK, SK) is adjustable, according to a system valve (4), and the secondary pressure circuit (SK) comprises at least one cooling device (8) for cooling the hydraulic fluid and a lubrication circuit (9) located downstream of the cooling device (8), between the cooling device (8) and the lubrication circuit (9), one bypass pipe (L5), blockable via a valve unit (VE), is provided which can be controlled via the valve unit (VE) so that a hydraulic fluid flow rate (Q_K) supplied, via the cooling device (8), is adjustable without changing a hydraulic fluid flow rate (Q_S) to be supplied to the lubrication circuit (9).