Yes, you will need some pressure to make it work.
I just want to point out one thing. Jim's car is the RARE case. Most cars with Klima relays use a ground signal from the push button controller. When working properly the voltage coming from the compressor relay is pulled to ground to activate it.
If one were to apply 12v to this circuit it will take out the PBC PC board. There are way more models that use ground signals than use 12v signals. Jim has a one year situation with the 85 300D. Earlier cars (such as yours)don't have the klima relay and the power to the compressor actually passes through the low pressure switch.
Just be carefull if you want to jump this circuit. (This is a general warning not specific to this case)
Bosch Master, ASE Master, L1
33 years MB technician