My 722.5 'box, once warmed up, would engage reverse 50% of the time. A year ago, I had box and torque converter (DON'T forget to drain the TC) fluid and filter changed by an MB specialist. MB filter and MB D3 fluid were used.

This was a high price and, in fairness to them, they advised me to forget about the fluid change once they'd drained the 'box and found bits of debris. I told 'em to go ahead anyway.

A year later, it hasn't failed once to engage reverse. £230 well spent, even if it fails today!

RayH