[Healeys] Austin Healey BN2 overdrive issue

[David Wirken]

I have a issue with the overdrive not disengaging and becoming stuck in OD after disengaging, it’s not an electrical  issue and we have to rock the car back and forth until we hear the clutch pop loose and off we go until I engage it again and then try to disengage it , I have pulled the operating valve and cleaned real good the tiny hole in the shaft and have drilled it out one size bigger as directed by a Healey restorer , to no avail.
Any helpful hints ?

DW