Coadman: That is the emergency brake drum at the input side of the differential. Been awhile. But I think you can take out the four bolts and slide the drum back to check brake lining. If not you will have to remove drive shaft. Brake shoes have return spring and they may have broke. Hand brake is mechanical, operates this brake. Make sure it is releasing? Your service brakes are operated by hydraulic in the drive wheels. Just like your car,from a foot operated master cylinder. Also check your drive wheels for locking up? Back tilt your upright or mast. Raise the carriage to get the forks out of the way. Place some oak wood if possible under the outer rails of the mast. Then tilt the mast forward. This will get both drive wheels off the floor if you have no jack available to get wheels off the ground. Put truck in neutral and release hand brake. Both wheels should turn freely? I one turns all most all the way around then locks? You may want to remove and check the bull gear & jackshaft inside? Axle has a gear on the end of it. And wheel inside has a ring gear on it. Some times bolts break and get stuck between gears? Good Luck ETS