Auto gearbox - after some advice

[cb750chris]

Hi All,
I was asked to look at a car recently which has stopped 'dragging' when reverse is engaged and the owner described it as 'reluctant to reverse'. Another symptom was that the ABS light was coming on intermittently.

My first thoughts were that this was possibly a failure of one of the rear backplates, causing the brake shoes to move and partially lock a rear wheel - with the rust from the backplate upsetting the ABS sensor. Or a second thought being a cracked reverse piston causing the gearbox to loose pressure, so needing a bit more revs to reverse.

Plugging in the car I got a different symptom to what I expected - The ABS fault was being caused by a front sensor - turned out it had been rubbing and a core of the cable had failed.
There were no recorded faults on the autogearbox (as would be expected from a duff reverse piston).

Jacking up the rear of the car showed both rear wheels rotated freely in both directions - so not a rear wheel issue (he also had a local mechanic check the rear shoes before visiting me).

However jacking up the front of the car (one side at a time). With the car in neutral (ignition off) the wheels turned freely forwards, however it was significantly more difficult to turn the wheel in the reverse direction (a lot more so than a different auto 75 that was available at the same time - this car was also more difficult to turn the wheels backwards - so I assume that is just a feature of the gearbox).

The wheel turned freely backwards a few degrees, until the driveshaft 'play' was taken up (suggesting to me it is not a front brake issue, the pads are nto dragging), at which point things went tight, and 'lumpy' - the best way I could describe it would be as though turning an engine over (albeit with very low compression)

Does anybody have any suggestions as to what I could be looking at here? It doesn't feel like any of the following:
1) Duff solenoid- I would expect the T4 to flag the fault, and there are no issues when driving forward, and the mechanical feel to the wheel turning.
2) Low/Dirty oil - Unless the gearbox is stuffed I wouldn't expect oil level to make difference when turning by hand with the engine off.
3) Cracked reverse piston - as I understand this failure is a simple pressure leak internal to the box, causing loss of clamping pressure, this shouldn't affect me turning the wheels by hand.
4) Car Brakes - as the wheels all turn freely in both directions, until the play in the driveshaft is taken up. The brake pads do not drag/catch.

I don't want to start changing things that don't need changing - I want to give the owner the best cost fix - he isn't too local to me, so I also don't want it turn in to a multiple visit solution either.

Thanks
Chris