This is a repost of part of another thread going on now, I think that the topic deserves it's own heading so I'm beginning anew.
Many people choose to fill the canister with water before reconnecting, in the hopes that excluding that air will make start up easier. This does not solve the problem for typically a bubble will be trapped below the valves and the bubble rising in the inlet line breaks the siphon or get trapped in the impeller and airlocks the filter. Most importantly, this method ignores the physics involved.
The whole thing to priming is to be sure the air is expelled the right way, through the outlet side of the filter, not up the inlet which will break your siphon. How much air is expelled is not important, as long as it moves the right way. That can be done, as was said in the other thread, by connecting the waterfilled inlet lines to the canister and letting air out the outlet side of the filter until water has filled the filter and runs out the outlet (have a towel or bucket handy.)
It can also be done by having the outlet tube completely empty of water when the unit is reconnected, but be sure the in-tank outlet/spraybar is above (or alt least very near) the level of water in the tank so air can escape. The weight of the water in the inlet tubes will assure you that no air can move up the inlet line, water will flow down that tube, air up the other. If both tubes are dry, perhaps you just cleaned the tubes, connect the inlet taps, suck on the filter outlet tap just to get water flowing into the filter then connect the outlet tubes before the filter is full, all the inlet side is full of water which is heavy, so air must go up the outlet. When rushing water stops you turn on the filter. If the inlet tube is still full, perhaps you bled water from the lines to clean them and the outlet is empty but the inlet is full, then simply connect all tubes, open outlet taps to allow air flow, then open inlet taps to begin water flow. Again, when the water stops rushing, turn on the filter.
I cut the &#%$@% out of my hand rocking a Fluval 303 trying to get it going following that bad advice to first fill the filter with water. I spent an hour on the stupid thing, running late for an appointment by then, and while rocking the filter to try to get the air out my knuckle hit the sharp glass edge of the lower tank. It bled a lot, a real gusher. So I finally had to sit and think about why it was so difficult. Looking that physics of the thing, it becomes obvious -- you gotta get the air to go up the outlet. Either bleed it out, past the impeller by letting air, then water, escape the canister from the outlet side, or have the entire oulet line full of air so that it all has to go up with water pushing it from the inlet.
So, for units that have the integrated taps but no self priming, you might need to bleed all water from the outlet line -- I have begun doing this as part of my procedure, it flushes a lot of crud from the lines that otherwise can flood into the tank and it gives me some tank water in my bucket to rinse media in.
I find that the instructions on the canisters I have show pictures of the outlet lines (spray bars) above the water level of the tank when starting the filter, but they don't state that is necessary for air to get out of the system. Even the self-priming units can have difficulty if the outlet is below water when starting up if air needs to get out of the system.
If you are having trouble starting a canister -- look to the path the air is taking. It is the key.