One problem related to brake master cylinder has been reported for the 1997 Chevrolet Astro. The most recently reported issues are listed below.
Just before stopping at an intersection the brake pedal lifted slightly. There was a moaning sound from under the hood on the driver's side and the van drifted into the crosswalk ( lost braking ). I threw it into reverse to stop. After replacing the brake master cylinder, hydro booster, power steering pump and gear and hoses, the problem persisted. I finally found someone with the same problem on an on-line forum who suggested disabling the abs by pulling the fuse. Problem immediately went away when anti -lock brake was disabled.