The invention provides a pump of the type comprising a body (1) having a bottom portion for communicating with a dip tube (2) via an admission valve and a top portion for connection to the edge of a container (R) and itself containing a metering chamber (10) closed in leaktight manner on top by compression means provided with an exhaust valve and with a return member, and suitable for being actuated by manual thrust being applied to a dispenser head (T) provided with a delivery duct (C), the pump being characterized in that said return member comprises an elastically deformable internal diaphragm (3) carried by the dispenser head (T) and co-operating with a rigid cup (4) fixed beneath said head (T) and co-operating with said diaphragm (3) to define the metering chamber (10).