[TR] TR4 overdrive

[dave]

I'm assuming it needs an overhaul.

Topping off fluid makes no difference (it really wasn't low)
When I have the car turned on but not running, in 4th gear, engage OD with
the car on jack stands I hear the thunk

Almost never engages; sometimes when I let off the throttle it seems to drop
too much, then rev higher than expected when pressing it back down.

It used to work . . .

Early TR4 - 1962, original overdrive transmission I think.  I'm third owner;
I've had the car operational for about 2 years now.  I found it languishing
in a barn where it had sat for 25 to 30 years.  Rebuilt engine and carbs,
redid all the hydraulics, replaced all the fluids, etc - all the stuff you
have to do with a sleeping car.  fixed a few other things here and there.
Car drives fine and the overdrive worked great.

Obviously I don't know the history of the transmission, other than the car
originally did come with overdrive.  Tran shifts just fine; but I seem to
have lost the overdrive in the last few months.  Haven't taken the
transmission tunnel off yet.

Any other tests I should do?