It's just that you have a bigger problem than getting that pvc
tube going. I guarantee you that you will run into either of two problems:
1) You will overflow your prop tank because the pump isn't going fast enough, causing your tank to drain all over the place until it reaches the bottom of the pvc
tube to let air into the system, or
2) You will burn up your pump because it's pumping too fast and you're draining your prop tank too fast.
I thought I could do something like this too, but trust me, this kind of setup is just asking for trouble. If you do want to try it for yourself, I would start the syphon by installing a "T" joint with a threaded end on the T, and a male threaded cap ready to go. Put the T on the way down to the prop tank. Start to suck the air out while plugging up the exit hole of the pvc
tube with your finger (inside the prop tank), and when it starts, quickly plug up the open end of the T by screwing in that cap. Just make sure to have a towel handy cuz it'll spray all over the place till you get that cap on. If you put a ball valve above where the T is, you can also shut off the flow before the water runs all the way down, and after the cap is on, you can open the ball valve the rest of the way.