The part you need is the control pressure cable itself. It comes with the vacuum element made to the transmission side. If the line from the switchover valve won't hold vacuum (and is secure at both ends) then this is what is leaking.
The part # is 124 270 22 73 and is an updated part needing part # 000 987 11 45 to plug off the second vacuum line.
The timing is decided inside the control unit (it switches the switchover valve and applies vacuum to the cable element. In the case of the C220 it is just a fixed interval (I think). On others it only takes place below an engine temp and then for an interval.
Bosch Master, ASE Master, L1
33 years MB technician