Model 755 18 hp diesel. Brakes sticking

Please raise the machine so you can work under it and examine the brakes.
Brakes often can stick from rust, dirt, vegetation from mowing, etc.
It might be you need to do a good cleaning, oiling, and adjusting of your mechanisms.
God bless your efforts.

you may receive more help if you list the manufacturer, model, and details such as it is a hydrostat, engine size (18 HP or 22 horse briggs motor). also describe your symptoms a little more as in: it ran fine last week then today when I started mowing it just didn't want move in reverse, or was mowing when it just quit working.Hope this helps.

are your blades engaged when you try to do this??
if so it is a back up safety feature from factory,turn blades off and try?
Or put the ignition switch in the position that will keep the mower running when you go in reverse with the blades on or push a button on dash when you go in reverse,probably not on yours just some of the features on some models and just something to look for on yours

Make sure your safety switches are all closed like the mowing deck disengaged, sitting on seat, neutral etc.

The brake lever has a hole in it as you must have found the other end attaches to a finger bracket that sticks up from the mow deck.

the brake mechanism is likely stuck, if you pull the hydro release rod and attempt to push the mower it should free wheel reasonably well, if it continues dragging then the brake shoes are likely seized, even though the arm may be free

most likely the blades are still in the on position,but you have a seat switch, parking brake switch, and the blade switch, also the mow in reverse key switch. if mower dies once you let out the clutch in reverse with blades on and key wrong.

Check for a broken or dislodged belt

if it has a reverse belt it could be broken or sliped off the pulley