Just to add to S Steve's post, think my memory is correct.
Ref. No back feeding of APU bleed air.
If right eng bleed shows not "off " and no APU bleed avail, X bleed to closed and APU bleed comes on ok, X valve to open and and duct px will bang the right engine bleed to show "off "
If left eng bleed is problem, just select any eng start, when you get duct px for a few seconds, switch off selected eng start and all should be ok.
Note of caution, selecting start switches should be done with care, a little switch can cause many problems for those who choose to do this quick fix to overcome the bleed logic on the 757 when eng bleed valves fail to show the "off " indication. Take Care !!!!