This invention relates to an automatic adjusting device for hydraulical working pistons, comprising a threaded spindle onto which an adjusting nut limiting the return movement of the working piston is screwed, a frictional-engagement turning mechanism for the adjusting nut, the operation of the turning mechanism is dependent on the stroke of the working piston, a brake for preventing a turning movement of the adjusting nut, the operation of the brake acting on the working piston is dependent on the hydraulic pressure, and a resilient connection between the turning mechanism and the working piston.  Such an adjusting device is known from the German published patent application DT-OS 2,330,949. 
It is an old problem of automatic adjusting devices that after the necessary lifting clearance has been overcome, the additional actuation stroke occurring due to pad wear is to be adjusted, but not the additional actuation stroke resulting from elastic elongations in the brake.  The latter would prevent release of the brake after the brake actuating process in case the advantage of a small lifting clearance is not renounced.  Therefore, the adjusting device must distinguish between a stroke of the working piston, which is to be adjusted, and a stroke which is not to be adjusted also after the lifting clearance has been overcome. 
In adjusting devices wherein the turning mechanisms for the adjusting nut include an adjusting sleeve with a slanting slot (see e.g. German published patent application DT-OS 1,902,597), this difficulty can be solved in that the slot is designed in such a way that with a large stroke of the working piston, only a part of this stroke is adjusted.  This design has the disadvantage that no constant lifting clearance can be maintained and that the lifting clearance becomes too small after repeated brake actuation, which might prevent a release of the brake in extreme cases. 
The adjusting device according to the initially named German published patent application DT-OS 2,330,949 solves this problem by means of a friction brake which prevents the adjusting nut from turning above a certain hydraulic pressure and, thus, realizes a constant lifting clearance.  But up to now, the friction brake could not prevail in practice because it is considerably more complicated in design relative to usual adjusting devices.  Adjusting devices for hydraulically actuated disc brakes are necessary in case disc brakes shall be combined with a mechanical actuation.  The high costs for the adjusting devices led among other considerations to the fact that drum brakes are provided for vehicles at the rear wheels, which drum brakes can be mechanically actuated without an adjusting device.  The friction brake according to the German published patent application DT-OS 2,330,949 is formed such that the threaded spindle of the adjusting device has a coaxial bore in which a piston is sealably guided.  This piston is actuated by the atmosphere pressure, on the one hand, and by the hydraulic pressure in the brake, on the other hand.  The piston is prestressed by a strong spring in the direction towards the chamber in which the hydraulic pressure of the brake acts.  The hydraulic pressure in the braking system effects a movement of the piston against the force of this spring.  In this process, the piston shifts a radial detent moving into the thread of the adjusting nut of the adjusting device and preventing the adjusting nut from a further turning.