A pre-compression system for liquid dispensing devices, such as sprayers, is presented. The pre-compression system includes a pump chamber and a valve chamber including a pre-compression valve member. The valve member is operable to allow liquid in the pump chamber to exit via an outlet only after a predetermined pressure is reached in the pump chamber, and to stop liquid from exiting when the pressure in the pump chamber falls below the predetermined pressure. The valve member includes an elastic diaphragm and a valve seat. The valve member has a concave surface facing a valve seat opening and a convex surface in communication with atmospheric pressure.