A dispenser pump has a plunger operable in a body including an outer cylinder body having a vent opening in its sidewall and a body insert fitting into the top of the outer body and providing an external collar through which the plunger stem operates. The body insert has lock-down threads to lock down the plunger for shipping. The insert also has formations for blocking or unblocking the vent opening by rotation between the outer body and body insert, driven by rotation of the plunger head which makes a catch engagement with the top of the body insert. This plunger head rotation is the locking or unlocking action for the plunger lock-down. Blocking the vent during shipping reduces leakage.