Filtration-compression cells are usually used during laboratory tests to describe the behavior of materials submitted to mechanical dewatering processes. Among the parameters measured, the liquid pressure at the surface of the filtration cake is a good indicator of the dewatering evolution. However, in the classically designed cells, the information given by pressure transducer during the expression phase does not confirm the theory. A new design of the piston is proposed and studied in this paper. A pseudo piston is added ahead of the pressure transducer in order to avoid the contact with the solid forming the cake during the expression stage. Experiments were carried out on different compressible materials: talc, kaolin, and activated sludge. Every experiment showed that the liquid pressure at the surface of the cake reached zero at the end of the dewatering test, even for extremely compressible cakes.