Actually I think that the regulator is set at 8 PSI,also Prevost unloads the tag when the front slide is moved and that takes a good deal of air and yes it should be in the normal drive position.I think that there is a vacuum generator to help get the air out of the seal,I might be wrong about the Vacuum generator.I know that there is a lot of compressed air used in moving a front slide,the rear slide does not unload the tag.