I'm sure someone would be along to correct me but I would remove both pipes from the tank and let the pipes drain, undo the filter as I'm guessing your changing that too, blow down the supply hose until nothing comes out the return hose.

Replace filter and all pipes to the tank, take off the return line at the charger end, blow down the return line to put pressure in the tank to bleed the charger system, wait until a constant flow of fluid comes out of the charger (make sure the tank always has fluid in it), connect return line on charger and top up the tank to the NEW required level as shown on the rotrex pdf file!

An amendment came out a while ago showing different "empty" and "full" marks apposed to what your dipstick will show