A control device for a braking system of a vehicle, with the aid of which at least one provided setpoint variable regarding a setpoint brake pressure to be set in a first brake circuit and at least one provided actual variable regarding an actual brake pressure that is present in the first brake circuit, and/or an elasticity variable regarding an elasticity of a second brake circuit of the braking system are receivable, and a default variable regarding a setpoint additional volume of a brake fluid of the second brake circuit which is in a main cylinder braking mode is establishable, by taking into account a first deviation variable of the at least one actual variable from the at least one setpoint variable, and/or a second deviation variable of the elasticity variable from a predefined setpoint elasticity variable, so that a control signal corresponding to the established default variable is outputtable to at least one volume conveying device. Moreover, also described is a method for operating a braking system for a vehicle.