In a liquid pump for a braking system of a vehicle, one end of a piston is slidably inserted into a cylindrical space of a housing member, and a sliding ring is provided on the piston. A suction chamber is formed by an inner peripheral surface of the housing member, an outer peripheral surface of the piston and the sliding ring. An inner diameter "Dk" of the housing member is made larger than an inner diameter "Dc" of the cylindrical space. As a result, the volume of the suction chamber is increased, without making the piston smaller in its diameter.