A main brake cylinder, for use within an electronic braking system for motor vehicles, having a device for contactless monitoring of a position of a piston that can be linearly moved along a piston axis. The device includes a movable element and a sensor unit. The sensor unit is fixedly arranged on a housing of the main brake cylinder and is connected to an electronic evaluation unit for processing signals from the device. According to the invention, in order to further increase function precision, to lower manufacturing expenditure, number of components, and costs, to reduce overall length, and to improve calibration options, a magnetic field is realized such as to be emittable from the sensor unit, and a yoke is dedicated to the piston for position-dependently changing the magnetic field such that the change can be detected by the sensor unit.