an outwardly projecting inner part 18b while the fitting 9b forms the socket 17b for engagement of the inner part 18b fig4 shows between the adapter 8c and the fitting 9c a plug type connection generally designated by reference numeral 16c and formed by an outwardly projecting inner part 18c which projects out in one piece from the fitting 9c and is received in a socket 17c of the adapter 8c the plug type connection 16c is complemented by a safety mechanism in the form of a snap on connection generally designated by reference numeral 24c and including two retention elements 25 jutting out from opposite sides of the fitting 9c and extending axially toward the adapter 8c the retention elements 25 terminate in retaining lugs 26 which extend radially inwardly and are capable of hooking behind complementary undercuts 27 formed on the fitting confronting end face of the adapter 8c in order to realize a biased i e force fitting arrangement of the fitting 9c in the adapter 8c a spring element 28 is supported on the end face of the inner part 18 the spring element 28 is preferably a disk spring 28 and rests on a shoulder 29 formed by a counterbore of the bore 30 for conducting hydraulic fluid through the adapter 8c in fig5 the adapter 8d is coupled to the fitting 9d via the plug type connection 16d which substantially corresponds to the plug type connection 16c in fig4 i e the fitting 9d.