Mike, there is some parts commonality between your 280 series and other GMC same year, different model trucks. I'd suggest contacting 'Bolter SlimSix and purchasing his 1939-1953 GMC Master Parts Book .pdf scan which covers 100 thru 350 models.
Below is the page from that manual that deals with rear brakes on 280 models. Not only are there two wheel cylinders for each rear wheel on your 280, there are different parts used on 280s with serial number 2863 and lower vs. 2864 and higher. The good news is that the same wheel cylinders were used with 280, 300 and 350 model trucks. The better news is that the first two are available on ePay, the third from either Dorman or Chevs of the 40s and the last through all those places.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12 '52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
