To bleed the system you just need to cycle the hydraulics fully a bunch of times. All the way up, All the way down, over and over again. It may take about 10 minutes of doing this, but eventually it will bleed out. The negative side of this is that the down position is not a hydraulic pressure down, so you are relying on the weight of the mower deck to compress the cylinder. If things are bound up or rusty sometimes going down takes quite a while, and/or if it is bound up, it may not be going fully down.
Also, check the depth stop on the left side of the tractor. big 3/4" bolt about 6-inches long that threads down next to the frame near your left foot. Here is a picture;
This is how you set your height of cut, so every time you let the mower down it stops at the same point. If it is set for a hight cut, you are probably not letting the cylinder down very far at all. The bolt acts as a stop against the rockshaft, limiting the rotation, and thereby stopping the mower deck from lowering. I know that when I set mine up for mowing, the mower actually only drops about 1/4", because I like a really high cut. However if you back this bolt out, the mower deck will drop right to the ground.