valve ii and continuously measure the pressure using pressure sensor 26 to determine when for an initial pressure rise the current amplitude sa 1 of the primary valve is reached i e the primary valve is opened and connects the corresponding branch line to the corresponding air bellows which can be recorded from the initial pressure increase 5 pressurize the gallery line 12 with the primary valve open e g to 10 bar 6 vent the gallery line with the primary valve open and continuously measure the pressure using the pressure sensor 26 7 reduce the current amplitude sa and continuously measure the pressure using the pressure sensor 26 as a result the current amplitude sa 3 of the primary valve i is reached if the pressure does not reduce further a pressure that does not reduce further is the result of the primary valve i changing from the open switching state into the closed switching state the current amplitudes sa 1 sa 3 of the switching currents of the primary valve are determined by steps 4 and 7 8 with the switched current amplitude sa 1 and the gallery line 12 and the air bellows thereby connected via the primary valve i vent once to e g 1 bar 9 increase the current amplitude sa further and continuously measure the pressure using pressure sensor 26 if a pressure increase is observed the current amplitude sa 2 is reached to switch on the secondary valve ii because the secondary valve ii opens this is noticeable.