Eheim canister filter?

I bought an Eheim 2215 canister filter for my 77 gallon tank and can't figure out how to get it working. When I try to prime it, it will not suck water from the tank into the filter. I have done everything according to the instructions...I think! I have even tried switching the suction hose and pressure hose connections, and turning off the taps on either hose. Nothing seems to work. Am I doing something wrong or is the filter defective?