Hi Alex,
I do not think they always make a perfect seal, so I wouldn’t use them to try to e.g. create a permanent vacuum. However, I think that they would work for the application you outlined: I expect that by running the output pump continuously you would generate some amount of suction (it doesn’t sound like you need much for your application), which would draw in air from another pipe.
I come to this conclusion because I have observed that if you block the input end of the pipe with your finger you can observe the motor struggle, implying that it is able to create a fairly good seal and generate suction.
.
