I had a similar problem with the front wheel bearings on my Heavy Duti (same rear brake as yours) It turned out that the front wheel uses a 15/64th ball, and not a 1/4. I'm not sure, but this might be the case with your rear bearings. I had to use a drill index to sort through all my old bearings in order to find enough of the smaller ones to complete the bearing set. The 1/4 " bearings wouldn't drop through the hole that held the 15/64th inch drill bit. If you have new bearings, see if they are the exact size as the old ones. If you don't have a drill index that goes by 64ths, put some of the new, and older bearings together between two pieces of metal, or glass to see if the new ones are larger. It's almost impossible to tell by just looking. Hope this helps.
Again, it might be easier to just find a new brake altogether.
JWM