Disconnect hose from the backside of the toilet. That will let you suck out at least.
I've cleaned my tank by pumping hard on the toilet while the vacuum was connected when my vent line plugged, flushed tank with sea water, then disco toilet output hose while tank is under vacuum from the pump
, maybe a little dribble of sea water.
Less messy, and you could replace all head hoses at the same time to extend the life of everything and not be working with a full tank.