767 tech manager reply
I asked one of our tech Managers for our 767 fleet. He indicated that it’s very important to close the engine bleed switches before engine shut down and to close the APU shutoff valve before APU shutdown.
He said the engine bleed switches control the Pressure Regulating Shutoff Valve (PRSOV) which is regulated by balancing a differential between upstream and downstream air pressure. When the bleeds are shut off before the engines are shut-down there is enough duct pressure to ensure sufficient muscle pressure in the regulating lines to close the valve completely. When this is not done there is a chance, through normal wear of the piston spring and ring wear, that the valve may not fully close, resulting in a failure to illuminate the ENG BLEED OFF LIGHT. This results in log write ups for BLEED WILL NOT CLOSE WHEN SELECTED OFF on ENGINE SHUTDOWN. Many of these write ups result in No Trouble Found, with Ops Checked good Maintenance write-ups.