Activate pump 1, return to main room, load the pump 2 level, go back to the main room and continue through to the next area.
Found here http://forums.blackmesasource.com/showthread.php?t=14135
Wow. That is awesome. This game is gonna be great for speedrunning.
“This video is unavailable. Sorry about that”
Aww 
y u do dis
not my fault