Since it only happens when turning the wheel while backing, when was the last time the rubber brake lines were replaced? Possible they could be breaking down internally and the added stress of turning the wheel prevents fluid flow.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.