Vapor lock on in tank pump?

Even if it was possible to get a vapor lock in a constant flow system, you still would have been showing pressure at the gauge (Air pressure, but pressure none the less)
I'd bet ya it's the pump or power supply to it.

Hmm, not sure how the pump is connected since I bought the whole tank as a unit. My guess is that it's just a rubber hose clamped on both ends so that you can replace the pump fairly easily. It's an Aeromotive adjustable regulator with a vacuum reference. It started up fine after sitting for a while and drove around like nothing was wrong yesterday, so I don't think it would be a broken line or I would think it wouldn't be working at all. When I went back to it the day it messed up after a few hours and keyed it over it went right up to full pressure again, which is why I thought maybe vapor lock since it was a temporary problem (I didn't touch anything between letting it sit and going back to it later), but I'd never heard of that on a high pressure in tank pump.