Primming the eheim never a problem for me. I usually disconnect the outlet pipe from the spraybar (am using the new eheim installation set 2, a lot easier, just unscrew the quick disconnect) and put the end in a bucket. Then let water run through for a while, until the canister is full, then close the eheim quick disconnect valve, connect back the end pipe to the spraybar. Never need to push the primer knob (pump) either. As for the flow deteriorates, also never had any, since the external diffuser is connected to the outlet.

Thanks