There's a special fitting that was used to connect the steel tubing to the manifold. If you don't have it go to the JY. The steel tubing inserts into that fitting. Same thing on the modulator end.
The rubber only really serves to mechanically maintain the seal between the tubing and the fitting/modulator.
Since the rubber can collapse inward, it should have no exposure to vacuum. That is why the actuating line is entirely made of steel - as opposed to running a length of rubber hose down to the modulator. These images are bad but they might help explain:
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