It was actually for diesels, diesels don't have a throttle valve and therefore no manifold vacuum.GM discovered (?) that some cars didn't have enough vacuum to run all the HVAC and the brakes so they built/found a hydraulic gizmo that uses pressure from the power steering pump to power the brakes. Thye use it on the diesel stuff that has no vacuum too.
Try this:
http://www.hydratechbraking.com/
:cheers: