A precompression pump has a cylindrical housing 1 containing a slidable annular plunger 3 controlled by a push-rod 40 which slides in the plunger, and includes an outlet passage 41a, 42a leading to the housing via a side opening 42b. The plunger has an axial annular lip 4 that interacts with the push rod to block or clear the side opening, the push rod being connected to the plunger by a spring 47 which pushes the plunger back into a position in which the side opening is blocked. The spring urges the plunger towards an annular ram 44 which urges the lip 4 radially towards the push rod 40 by wedging it. The ram is configured to abut the plunger 3 outwardly of the lip and at a site distinct therefrom, thereby limiting movement of the ram 44 and the plunger towards each other, and thus limiting the radial force applied to the plunger lip 4.