The one way check valve is on the jurassics, classics and NGs. If memory serves me the check valve is calibrated to the maximum duct pressure so (in theory) will prevent backpressure from reaching the APU. So, LEM your question is valid why limit engine thrust to idle if the check valve is in place. In addition, the DB light illuminates if engine bleed switches and APU bleed value is configured in a manner so as to cause a dual bleed situation. So far so good....However, I have never understood why the checklist does not require the APU to be shut off in the event of a dual bleed light at any time other than engine starting. i.e. in flight. In other words, is placing the APU bleed to OFF sufficient to stop the problem? Why not place the APU switch to off? That would surely fix the problem or are Boeing engineers telling us that after shutting APU bleed if the Dual Bleed light remains illuminated the indication/warning is spurious.
Thanks,