An improved precompression metering-proportioning pump, said pump enabling a product to be discharged under pressure through the cooperation of five cylindrical parts having a common axis, namely:
- a) a hollow coupling-sleeve (1),
- b) a hollow casing (2) having two open end, said coupling-sleeve being secured to said first end (21) of said hollow casing, and a hollow cylinder (24) having an open section extending from said second end (22) of said hollow casing,
- c) a first piston (3) mounted to slide in an airtight fashion inside said hollow casing between a first and a second position, said first piston being applied against said coupling-sleeve in said first position, and having a hollow rod (31) extending through said first end of said hollow casing and having a valve seat (32) on its inside thereof,
- d) a differential piston (4) mounted to slide inside said hollow casing and comprising a shoulder (44) facing said first end of said hollow casing, said differential piston being integral with a valve spindle (41) which extends towards said first end of said hollow casing and which is engaged in said hollow rod of said first piston so as to cooperate with said valve seat, said differential piston being integral wit a skirt (42) which extends towards said second end of said hollow casing and having an outer guiding surface and an inner surface provided with an indentation (45) and a sealing lip (43), said skirt engaging said hollow cylinder (24) such that said sealing lip fits around said hollow cylinder (24) at least as soon a said first piston leaves said first position, said second position of said first piston being reached when said open section of said hollow cylinder comes to rest against said indentation of said inner surface of said skirt, and
- e) a return spring (5) disposed between said differential piston and said second end of said hollow casing,
- said differential piston defining at least one bore (63) extending from said inner surface of said skirt to said shoulder, a cylindrical sealing element (60) fitted around said valve spindle and being adapted to be applied against said shoulder level with two annular contacts between which said bore opens to form a non-return valve at the opening of said bore, and an open space (35) around said shoulder so that said sealing element can detach from said shoulder, wherein, when said pump is at rest, said sealing element is kept deformed so that it is applied against said shoulder with sufficient force to guarantee a seal against said two annular contacts.