The pneumatic line of the fischertechnik construction toy system has a manual valve which unfortunately does not allow any connections with standard fischertechnik parts like building blocks or axles. Thus, they are confined to manual usage.
This design replaces the blue part of the valve and enables a direct connection with standard fischertechnik building blocks or clip axles. Be warned that you'll have to take apart your original valve.
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You'll need a very accurate print to make this work, the problem is that you'll want to have enough pressure on the silicone ring to get it airtight and not break the axle clip at the same time.
If there is not enough pressure to get it completely airtight, you can try to insert a tiny wedge, cut from a toothpick or similar, in the gap at the backside.