I was thinking about that too, but if the manual valve was between two gears,nothing happens. Plus the second gear oil-circuit is pressurized; in both second and third. It's just that when the manual valve calls for third oil pressure is supposed to simultaneously force the second gear servo piston to the park position which relaxes the band.
Oh I think I know whats going on; Maybe the hi-drum piston seal blew out. That would cause the oil pressure in the hi-drum to be lower than in the servo. The servo might remain engaged while the hi-drum slips. Yeah that might do it. It's the best I got, and the air-pressure test will prove it.
