A precompression system (7; 107; 207) for a liquid dispensing device (1; 101; 201) having an inlet (5; 105; 205) and an outlet (6; 106; 206), the precompression system (7; 107; 207) comprising: a pump chamber (15; 115; 215) including a piston (16; 116; 216) movable in the pump chamber (15; 115; 215) for drawing liquid through the inlet (5; 105; 205) and discharging the liquid through the outlet (6; 106; 206); and a valve chamber (26; 126; 226) including a valve member (27; 127; 227) disposed between the pump chamber (15; 115; 215) and the outlet (6; 106; 206) and being operable to allow liquid in the pump chamber (15; 115; 215) to reach the outlet (6; 106; 206) only after a predetermined pressure is established in said pump chamber (15; 115; 215) and to stop liquid from reaching the outlet (6; 106; 206) when the pressure in the pump chamber (15; 115; 215) falls below said predetermined pressure, wherein the valve chamber (26; 126; 226) has an inlet end (28; 128; 228) in fluid communication with said pump chamber (15; 115; 215), an outlet end (29; 129; 229) in fluid communication with the outlet (6; 106; 206) and an annular valve seat (31; 131; 231) arranged between the inlet end (28; 128; 228) and the outlet end (29; 129; 229) and having an opening (30; 130; 230) extending therethrough, wherein the valve member (27; 127; 227) comprises an elastic diaphragm (32; 132; 232) normally closing the valve seat opening (30; 130; 230) in a closed state, a first side (32A; 132A; 232A) of said diaphragm (32; 132; 232) facing the valve seat opening (30; 130; 230) and being in fluid communication with the pump chamber (15; 115; 215) and a second side (32B; 132B; 232B) of said diaphragm (32; 132; 232) in communication with atmospheric pressure, and characterized in that said first side of said diaphragm (32; 132; 232) has a concave surface (32A; 132A; 232A) and said second side of said diaphragm (32; 132; 232) has a convex surface (32B; 132B; 232B), and in that said valve member (27; 127; 227) is stretched and prestressed in said closed state and is arranged to move away from said valve seat (31; 131; 231) to an open state when further stressed by at least said predetermined pressure. a pump chamber (15; 115; 215) including a piston (16; 116; 216) movable in the pump chamber (15; 115; 215) for drawing liquid through the inlet (5; 105; 205) and discharging the liquid through the outlet (6; 106; 206); and a valve chamber (26; 126; 226) including a valve member (27; 127; 227) disposed between the pump chamber (15; 115; 215) and the outlet (6; 106; 206) and being operable to allow liquid in the pump chamber (15; 115; 215) to reach the outlet (6; 106; 206) only after a predetermined pressure is established in said pump chamber (15; 115; 215) and to stop liquid from reaching the outlet (6; 106; 206) when the pressure in the pump chamber (15; 115; 215) falls below said predetermined pressure, wherein the valve chamber (26; 126; 226) has an inlet end (28; 128; 228) in fluid communication with said pump chamber (15; 115; 215), an outlet end (29; 129; 229) in fluid communication with the outlet (6; 106; 206) and an annular valve seat (31; 131; 231) arranged between the inlet end (28; 128; 228) and the outlet end (29; 129; 229) and having an opening (30; 130; 230) extending therethrough, wherein the valve member (27; 127; 227) comprises an elastic diaphragm (32; 132; 232) normally closing the valve seat opening (30; 130; 230) in a closed state, a first side (32A; 132A; 232A) of said diaphragm (32; 132; 232) facing the valve seat opening (30; 130; 230) and being in fluid communication with the pump chamber (15; 115; 215) and a second side (32B; 132B; 232B) of said diaphragm (32; 132; 232) in communication with atmospheric pressure, and characterized in that said first side of said diaphragm (32; 132; 232) has a concave surface (32A; 132A; 232A) and said second side of said diaphragm (32; 132; 232) has a convex surface (32B; 132B; 232B), and in that said valve member (27; 127; 227) is stretched and prestressed in said closed state and is arranged to move away from said valve seat (31; 131; 231) to an open state when further stressed by at least said predetermined pressure.