element 22 acts on an adjusting element 18 during the translatory motion the adjusting element 18 is shown in fig2 and is designed as a pivotable lever arm 28 furthermore a transmission element 30 which is in an oblique position to the plane of the pushing element 22 in an upper position of the lever arm 28 is formed at the pivotable lever arm 28 pushing element 22 acts on the slope of the transmission element 30 during a motion between the two switching positions 24 and 26 the transmission element 30 is arranged at the lever arm 28 such that lever arm 28 presses a valve spring 20 during the motion of the pushing element 22 in the embodiment shown in fig1 the pushing element 22 is in the first switching position 24 the valve spring 20 is in a relaxed state in the first switching position 24 and generates only a minimal prestress on the expiration valve 16 the first switching position 24 corresponds to the u201c normal pressure u201d mode fig3 schematically shows a design of the pushing element 22 the pushing element 22 is designed as an elastic double clasp 34 the elastic double clasp 34 comprises two lateral webs which are connected to one another via a connection web 36 connection web 36 is made elastic the locking elements 32 are arranged at the respective ends of the lateral webs moving together of the two locking elements 32 on one side brings about the moving apart of the two locking elements 32 of the other side pushing element 22 is movably guided within cover 38 cover 38 has two side openings.