Look at how the brakes are actuated with a rod. Spray all the pivot points with a penetrating spray like PB Blaster, Kroil etc. I would suspect that the levers into the rear wheel drive housing are probably not rotating freely and you may have to coax them to move by tapping them with a hammer in both directions. Don’t get too aggressive with any thing but just keep working on them daily, spray, move back and forth…repeat until they are freely moving. Then lubricate them with a better lubricant.
You should grease the zerks on the pedal also.