Quote Originally Posted by spookmineer View Post
With the amount of headpressure pumps have, I don't think there's actually a vacuum at the suction side of a pump.
Ok, if we're being pedantic, there's pressure that is less than atmospheric pressure before the pump inlet, causing the atmosphere to push in and attempt to crush the tubing.

There have been numerous posts from various people where they've shown that with soft-walled tubing, all you need to do is to slightly restrict the run into the pump inlet and the tubing will collapse. Sticking a restrictive block before the pump inlet will achieve the same effect.

If you don't believe me, I'll set up a test and video it in effect for you, using a DDC-2. Believe me, it happens.