A hydraulic system of a vehicle is provided, said system including:a first hydraulic circuit (1) having a first pressure source (2) and at least a first hydraulic consumer (3, 4),a second hydraulic circuit (5) having a second pressure source (6) and at least a second hydraulic consumer (7),connecting means (10, 11) allowing a flow of hydraulic fluid at least from said first hydraulic circuit (1) to said second hydraulic circuit (5),and control means (16) for controlling operation of said hydraulic consumers (3, 4, 7). Such a hydraulic system should allow safe operation of a vehicle with low energy consumption of the hydraulic system. To this end said control means (16) are connected to summation demand means (12), said summation demand means (12) controlling flow of hydraulic fluid from at least said first hydraulic circuit (1) to said second hydraulic circuit (5), said summation demand means (12) being connected to operation mode signal means (17) outputting an operation mode signal indicative of an operation mode of said vehicle, wherein said summation demand means (12) allow or block flow of hydraulic fluid from one hydraulic circuit (1) to another hydraulic circuit (5) in dependency of said operation mode signal.