The proposal is for a power brake for motor vehicles, the control valve of which can be actuated by an electromagnet independently of an actuating rod moved by a valve piston. The electromagnet and other electrical means are powered via electric leads which extend through a grommet secured airtightly in the power brake housing. In order to prevent the unintentional release of the cable grommet (7) in critical safety circumstances and a concomitant breakdown of the power brake, according to the invention the cable grommet (7) is held in the power brake housing (1) by means of a bayonet fitting (9, 10 or 11, 12) and consists of a retainer (9) insertable into the power brake housing (1) from inside and a bayonet ring (10) acting together with the retainer (9) bearing on the power brake housing (1) via a seal (13).