Mr. Limpet said:
PSI? Maybe 1 or 2 would be my guesstimate.
There are 3 ways to get a gravel vac syphoning that I know of.
There should be a flapper valve that as you have the tube in the tank water rock it forwards and backwards (horizontally) it fills the hose to get the syphon going.
The next way is to dip the tube into the water and fill it, with a hand ready to plug the drain end of the hose, raise the tube up (vertically) until the hose is filled then stop the flow with your thumb before the tube empties. Put the tube back into the water till the tube is full of water, then let go of the drain end with your thumb and it should just start to flow.
The last way can be the nastiest if you aren't quick lol. Put the tube in the tank, fill it with water, then create a vacuum by sucking on the drain end of the hose until water is flowing.