The invention relates to a fluid control device (500) for treating a well, to expand a liner (100) radially outward, said device comprising a piston (550) translatably mounted in said chamber (320), and degradable immobilization means (590), secured in translation with the piston (550), wherein the degradable immobilization means (590) are movable in translation in a cavity (700), wherein the device comprises protection means (800) isolating the cavity (700) from a discharge pipe (810) configured to introduce fluid degrading the pin (590) in said cavity (700), such that, in the initial state, the protection means (800) protect said degradable immobilization means (590), the protection means (800) being configured to break when the pressure in the cavity reaches a threshold pressure gap (ΔPs) allowing degradation of the degradable immobilization means (900).