Well Isln. valves has something to do with the air distribution, through the aircraft using a series of interconnected ducts. Normally the left and right distribution ducts are "isolated" so that air from each engine is supplied to it's on-side equipment only. However, though the use of isolation valves, bleed air can be used from any one source to supply the entire system. Additionally, the isolation valves are normally open on the ground so that the APU can supply air to the entire system and for engine starts.
The flow lines drawn on the pneumatic panel can help the pilot visualize the flow of air in the system. The left and right isolation valves are normally closed after engine start so that the left and right systems operate isolated.
These isolation valves must be open to supply air for engine starts. The center isolation valve is always left open and is only closed by abnormal procedure.
The air driven hydraulic pump recieves air for operation through the center duct. If the center isolation duct is closed and the APU is not supplying bleed air, the center hydraulic demand pump will not operate
Pneumatic duct pressure can be monitored using the duct pressure gauge or display on the pneumatic panel. Each ducts has a pressure needle that registers duct pressure downstream from the isolation valves.
I hope I didn't confuse or "bore" you, but that's the most detailed image I can give you for as this moment...
Where did you find the check list?!!
hope it did some good this report on isln.valves