Syringe adapted to overcome a pressure resistance

A syringe for expelling a liquid medium for filtration, said syringe comprising:

• a first cylinder cooperating with a first piston having a first hollow piston rod and defining a first pressure chamber, the hollow portion of said hollow piston rod defining a second cylinder having a smaller cross section than said first cylinder and cooperating with a second piston having a second piston rod and defining in said second cylinder a second pressure chamber;
• a bore formed within said first piston connecting said first and second pressure chambers;
• a valve in said bore having a valve closure element and means biasing said valve closure element to open the valve upon an over-pressure in said second pressure chamber;
• an inlet/outlet attachment for the liquid medium opening into said first pressure chamber;
• an air inlet duct formed in said second piston opening into said second pressure chamber;
• a second valve that opens in response to an underpressure in said second pressure chamber during an air intake stroke of said second piston and closes in response to an air over-pressure in said second pressure chamber during an air pressurizing stroke of said second piston, said second valve being in communication with said air inlet duct;
• a cover for said first cylinder;
• a stop on said first piston which is able to abut said cover in the extreme intake stroke position of the first piston;
• and
• with said liquid medium charged into said first pressure chamber by means of an intake stroke of the first piston being expelled by means of an air over-pressure provided upon said liquid medium within said first pressure chamber by means of a pumping action of said second piston.