The high stage valve is closed during engine start.(737NG).
As I say, my reference is to the original arrangement with the -100/200 PW-JT8 powered 737.
There's a very old sketch
here from a website discussing the 727 about 3/4 down the page. The bleed valve is not shown but clearly it would have to be open for engine start to allow air flow to the starter.
Hope this clarifies my point.
