Some embodiments are directed to a positive displacement pump comprising: a pipe having a first end secured to a transmission area and a second end ending in a cylinder secured to a discharge area, the pipe comprising a suction opening and the discharge area comprising a discharge opening, a drive shaft with one end situated at the cylinder, a piston pressed against the cylinder by elastic means so as to prevent fluid from moving between the pipe and the discharge area, in which the pump further comprises means for moving the piston elastically away from the cylinder and keeping it at a predetermined distance from same. Some other embodiments are directed to a method for cleaning this pump.