This is a common problem with the early 722.3,4,5, transmissions. MB had a problem with the clutch material. The reverse clutches flaked and loose their friction material, no reverse or a delay is the end result. When the wear is too much the piston rubs against the K1 drum, making a noise. Water contamination can also cause flaking; a strawberry looking coolant or transmission fluid is a sure sign of water contamination. Make sure any rebuild transmission you buy has the updated MB reverse frictions and steels.