Hydraulic system with suction/return filter

A thick-matter pump having two delivery cylinders (10, 10′) which contain in each case one delivery piston (22, 22′) and which issue at end-side openings (12, 12′) into a material-supply vessel and which, by means of a pipe switch (14) or a slide valve, are connected to a delivery line alternately during a pressure stroke of their delivery pistons (22, 22′), having two hydraulic drive cylinders (16, 16′) which contain in each case one drive piston (24, 24′) and the drive pistons (24, 24′) of which are connected to the delivery pistons (22, 22′) by means of in each case one common piston rod (26, 26′) and can be actuated and controlled by means of drive hydraulics (AH) that comprise a reversing pump (18, 20), and having a placing boom that can be actuated and controlled by means of boom hydraulics (MH), wherein the drive hydraulics (AH) and the boom hydraulics (MH) form a common primary circuit which has a tank (68), which is at atmospheric pressure, for accommodating hydraulic oil,

• wherein the primary circuit
• has at least one motor-driven primary pump (36, 38, 61, 71) that is charged with hydraulic oil via a first suction line (42),
• is connected at the outlet side to at least one first return line,
• and has a suction/return filter (40) which communicates at the outlet side with the first suction line (42) and which, at the inlet side, is charged with returned oil from the at least one return line,
• wherein at least one of the primary pumps (36, 38) is in the form of a charging and feed pump of the working hydraulics (AH) and a further one of the primary pumps (71) is connected to the boom hydraulics (MH),
• wherein a branch line is provided which is branched off from the first suction line (42) and which issues into the tank (68) via a preload valve (70),
• wherein a secondary circuit is provided,
• which secondary circuit has at least one motor-driven secondary pump (60, 62) which is charged with hydraulic oil via a second suction line (58),
• and which secondary circuit is connected at the outlet side to at least one second return line,
• wherein the second suction line (58) communicates with the tank (68) directly or via a suction filter (66),
• and wherein the second return line either issues into the tank (68) or is connected to the inlet side of the suction/return filter (40),
• wherein the first suction line (42) communicates with the tank (68) via a separate replenishment suction line (86) and a further suction filter (66), and wherein, in the replenishment suction line (86), there is arranged a replenishment suction valve (88) which is preloaded in the direction of the further suction filter (66).