GM Service Manual Online
For 1990-2009 cars only
  1. Apply the service brakes. Note the stoplamp operation.
  2. The stoplamp operation should occur with about 28-48 kPa (4-8 psi) application air pressure.
  3. If the stoplamp operation occurs too late, check the exterior light circuit. Refer to P92 Exterior Lights Cell 110.