I wouldn't worry about an air valve being left open quite as much as the vacuum being broken because a big chunk of concrete (or whatever) is now sucked into the tube in the path of the train.
Perhaps in the very unlikely event it ever rains again in California, water in the tube could be an issue, or lightning damage. That brings up the logical question of how many customers you'll have if none of them have any water to drink. Perhaps this would make more sense as a project under the Chicago Regional Transit Authority, or parallel to the existing Acela track on the coast.
No. Depending on the failure mode, maybe, but that is not the concern here.
A slow leak that the high-vac pumps cannot deal with is a slow deceleration.
A massive crack will cause quite the stir in the tube. Think of a balloon. When you inflate a balloon you put, maybe, 1.1 atms more into it. This is about 1/10th that of the hyperloop system. You know how loud even a simple party balloon can be, and the forces that you deal with.
Hyperloop is based on a reduced air pressure tube, not an evacuated tube. Additionally, there's inlets in the front of the capsules to actively move air/pressure from in front to underneath/behind.
I don't really know how that alters what you've stated, if at all, but it didn't seem to be addressed, so I figured I would throw it out there.
I would not call it gradually rising. Pods would be moving at huge speeds and then hit viscuos fluid (compared with vacuum)
It would be like hitting a wall. Pod should be constructed like supersonic jet to avoid crash.
Perhaps in the very unlikely event it ever rains again in California, water in the tube could be an issue, or lightning damage. That brings up the logical question of how many customers you'll have if none of them have any water to drink. Perhaps this would make more sense as a project under the Chicago Regional Transit Authority, or parallel to the existing Acela track on the coast.