As Brian says , straight air sets cars from the front. Slack action can be avoided by always keeping soem air in he straight air system . Since the straight air brake stand has no "lap" or "holding" notches ,it is hard to maintain an application . Automatic air sets from back to front ,keeping the trai stretched ,Retainers slow down the release , thereby assisting in minimizing slack . Sometimes it just boils down to the experience and skill of the engineer ,the condition of the brakes and the terrain . If the engineer feels the slack run in (usually a tap on his behind),it is throwing 'em on the floor in t he back !