I don't have a Fluval canister filter. Mine is a Rena Filstar, but most work in a similar fashion. As for the choke lever, that is to allow you to take the lid off without water siphoning all over your floor, and with mine, up is for maintenance time, down is for running. You should clean the filter material by preferably running it under dechlorinated water, though I don't because I have additional canisters and a deep under gravel filter with strong powerheads which give me lots of backup bacteria for cycling, and you can put charcoal in bags sold at aquarium stores for this purpose as well as other things like ammonia and phosphate absorbing chips in them as well. Hopefully yours came with media specifically intended to provide surface area for bacteria as well as the sponges mainly for particulates. Mine had the sponges but not the ceramic or plastic objects intended just for growing bacteria, but I bought mine new and it wasn't the Fluval which is supposed to be a really good one. Maybe if you must rinse the sponges under tap water you could just not rinse the bacteria promoting material as this shouldn't be clogged with gunk. This stuff should be placed in a compartment that gets water after it's filtered for particulates so bacteria can be free to attach and grow without getting smothered with guck. As for priming, I would fill the filter after cleaning with as much water as possible without making too much of a mess when you put the lid back on, attach the lid, put the lever down to open the circuit, and turn it on. If the water doesn't start running through the system right away, you may need to prime, and you do this with the filter turned off and the choke lever on the "start" position. With mine there is a screw cap I have to take off up by the edge of the tank where by water can be poured in till I've added what seems reasonable, no more than 1/2 gallon at most if you've filled the filter ahead of time. If the filter has no obvious way to prime you may not have to. My choke lever goes down to run it but I haven't looked closely at the Fluval. If water runs through it when it's up and not down you'll know. Nice acquisition!