overfilling the tank/hose will probably or not work because there is already enough fluid, or it will just cause the pistons to be further out and to make the pads rub against the rotor.
The best way is to adjust the levers from the adjustment screw next to them (normally a 2 or 3 mm Allen key hole fitting/screw, have a look if you have it. If you don't have it it means that your levers have been designed and produced only to have one position, and they believe is the best.
Never press the brakes when there is no wheel on because the pistons will start to come out, and eventually they'll be pushed out and also the oil; this way you'll loose the oil, have to quickly clean the caliper, and probably the pads will be now contaminated, and you'll have to change them.
The pads in fact soak in oil and stop to work/don't brake anymore, and the oil is highly corrosive.