the release lever is a rod sticking out the back of the tractor above the hitch. It needs to be pulled out, the rear of the mower raised off the ground, and then start the mower and move the lever, or pedals from forward to back to bleed the air out of the transmission pump components. then push the rod back in and see what happens.
when you had the tranny apart and replacing the springs, what did the center section look like(part the the pump and motor run on). The reject wear limit on the center section is, if you can catch a fingernail on grooves between the elongated holes, it is too damaged to use. when the springs break in the pump or the motor sections, pieces of the springs tend to grind grooves in the center section leading to what you are seeing.
The center section needs to be a machined flat surface between the oil inlet/outlet holes. If there is any grooving between the holes, it has to be replaced, along with the pump, and the motor, because the base of the pump and motor will also be damaged, and will destroy a new center sections within seconds.
As a Hydrogear warranty service center, I see center sections damaged quite frequently, due to people using there mowers for pulling loads too heavy, water infiltrating the transmissions due to pressure washing, and then freezing the water in the winter, breaking the pump springs, or in some cases people trying to change directions too quickly, causing what is called shock loading. the Hydrogear tranny operates at around 2200 psi, and quick directions changes in trannys without shock valves, will raise the pump off of the center sections, which drops the pressure, and then the pump slams down on the center sections, scoring the mating surface.