Hi you "insulation valve specialists"
If a ground air start has to be performed because APU is INOP there is a nice "trick"
if you are in MEX or similar place where the air is thin and the GND air equipment is sometimes really weak the ducktpressure is too low. maybe only 9psi
Remember that the demandpumps 1 and 4 are running on bleed, so if you turn them off helps a lot. its like closing a big hole in the duckt...
etc. Starting engine 3 and switching Dem. Pum 1 off and R ISLN makes your ductpressure rise from 9 to 25
its not an official procedure but might make your day....
Lars