front ABS brake caliper piston retract

I did wonder, but IIRC that GM was made in France.

That's not why I asked...

The (meager) evidence I've gathered is that it's the ZF that's less reliable.

Are you implying there's a difference in reliability between Steptronic and non Steptronic autos? Would seem unlikely to me since it's only a change to the control gear.

Some early 4 speed ZF did fail early when subjected to a US emission test which IIRC involved prolonged revving in neutral. But think this was fixed by the time the 5-speed arrived. Only 'straw poll' I have is that the 5HP19 as fitted to my 528 doesn't go for much on Ebay - so there can't be much of a demand.

I don't know that's the case. I think that Step=ZF. Maybe.

In the UK Steptronic was an option with the ZF box. I thought the same applied to the GM in the US.

AFAIK, it was an abrupt switch, on a per-model basis. Up to a certain time, if you got an auto with a certain model, it was not a Step, with no "add Step option" available. The Step auto was phased-in, not coincidentally about the same time that ZF was phased-in.

I have a fairly early E39 - '97 - and that has Steptronic. Think it was an option here from release. Of course in the UK there were base models not available in other countries - and plenty of those don't have Steptronic.

I thought all 6 cylinder E39s started off with the GM transmission in the US.

If it had a GM then it was not a Step. That's my theory that I think it true.

Any USA early E39 owners care to chime-in?