Do you have access to the Boeing Maintenance Manual and Wiring Schematics? (21-58-00 & 21-58-06 Schem 2 respectively)
"The inboard supply valve is installed behind the forward cargo compartment left sidewall, approximately 13 feet (4 meters) aft of forward endwall."
Some diagrams show it about 7 or 8 panels from the front of the cargo. Note that there are variations. e.g. Some wiring schematics show it at STA 470 WL 187 LBL 75 on later model -300's and at STA 515 WL 187 LBL 75 on earlier model -300's. Please refer to your own manuals for applicable locations.
Being in the sidewall, the air supply is cooled by contact with the (probably uninsulated) skin.
Rgds
NSEU
(EDIT) some pictures show the valve just forward of the forward cargo door ball mat area just below ceiling height (the inlet for the valve is aft of this, one panel behind, just on the forward edge of the ball mat)
Last edited by NSEU; 21st October 2015 at 02:12.