Its not a matter of being compressable. In an open ended system (access to different atmospheric pressures and not a vaccum). Take into effect.. If you were to have an open resevoir then turn the pump on.. the fluid level would go down using that pump. (If you had a close ended system full of water, the fluid wouldnt flow) As the water exits the loop into the open resevoir, the the vaccum of pulling is broken. The water is then at atmosheric pressure. Then it is sucked into the pump again increasing the pressure and this pressure goes down is it gets closer to outside atmopheric pressure.
I believe this is what Dave Smith was showing in his graph. But, I could be wrong.
pippen88's resevoir doens't seem to be a closed end system from the pics.
and mfpmax... I will tell you what you are looking at if you tell me what number I am thinking of.
LOL