Couplings/coupling heads dual-circuit braking systems are standardised acc. to DIN 74254 and can be combined as desired with other manufacturers. A distinction is made between coupling heads with and without valve. Coupling heads with valve are used on the tractors and prevent that after decoupling air leaks from the coupling. Couplings without valve are used on the trailer. The different brake circuits are differentiated from each other through yellow or red cover cap. The screw couplings constructed according to EDBRO dimensions are used as economical hydraulic couplings for automotive hydraulics and are widely used in the automotive industry.