The invention relates to a vacuum 
brake servo for a motor 
vehicle braking system, comprising a rod-like force input element which is coupled or can be coupled to a 
brake pedal, a control valve which is actuated by means of the force input element, a chamber arrangement having a 
vacuum chamber which can be connected to a vacuum source, and also having an operating chamber which, depending on the actuation of the force input element, can optionally be connected to the 
vacuum chamber and to the surrounding 
atmosphere. The control valve has a control 
piston that can be actuated by means of the force input element and, together with a valve element, forms a seal seat having a seal seat 
diameter between the operating chamber and the surrounding 
atmosphere. In the area close to the seal seat, a guide channel is provided, through which, when the seal seat is opened, air from the surrounding 
atmosphere flows into the operating chamber as a result of a 
pressure difference. In order to reduce the intake 
noise, provision is made in the guide channel for the latter to extend in the axial direction with a constant flow 
diameter over a predetermined length, wherein the flow 
diameter is less than or equal to the seal seat diameter.