diameter of the guide section 28 of the piston 20 should be approximately 5 to 10 smaller than the nominal diameter of the cylinder bore 18 at the time when the control device 10 is fitted and at ambient temperature this thereby pre determined initial fit play will decrease even further in time with the operation of the hydraulic control device 10 because as already mentioned at the beginning the guide section 28 of the piston 20 also undergoes plastic deformation upsetting and expansion under the prevailing operating pressures and temperatures as a result of which the sealing effect here is also improved 0049 fig1 to 17 show details of piston 20 of a second embodiment of the hydraulic control device 10 which will only be described below in respect of those features that differ from the first example embodiment as can be seen from fig1 in particular in the case of this embodiment the body 26 of the piston 20 has a shoulder 56 of smaller diameter following the guide seat 32 for the guide ring 30 via an annular shoulder 54 on which the grooved ring 36 is disposed the end of shoulder 56 of the body 26 which faces the pressure chamber 22 in the installed state of the piston 20 is provided with a radial groove 58 into which a preferably injection molded plastic ring illustrated separately in fig1 is inserted in this way the grooved ring 36 is held on the shoulder 56 of the piston body 26 between the plastic ring 60 and the annular shoulder 54 as is best illustrated in fig1 15 and 17 the outside perimeter of.