On the side of the tranny in front of the right tire, there is a slightly twisted piece of metal about 3 inches long, and what will look like a disc brake. Move that metal piece forward and backwards. Normally if it is stuck it will click loose when you try to move it in the disengage position. On some hydrostats the brake disc is on the inside of the tranny by will have that same piece of metal on the side. Most will either have a somewhat large spring hooked into the hole on the top. Others will have a rod that pulls or pushes the lever.
if that is the case where it is sticking you will need to put some lube on the mechanism while keeping it off the disc and the brake pads. In bad situations I have had to remove the brake system from the tranny and derust the pins that push the brake pad into the disc. Unless you know what you are doing I don't recommend trying it yourself. things tend to fall out and you will not notice them until later when there are spare parts laying on the garage floor.