Form all of the research I have done mine is truly the classic B2 piston partial failure. But I need to be positive.
Symptoms: Slips in first, barely gets started moving, but as speed increases, there is a more positive engagement (less slippage). Reverse is fine. It seems to have done this overnight. I serviced the fluid and filter, looks to have been done recently. Was thinking the valve body was sticking, disassembled, clean as a whistle. Reassembled, all same symptoms. Seems to be less slippage when cold, as a fluid leak???? more viscous when colder.
Another source said the motor and trans had to be removed to have access to the piston. If this is the piston, has anyone changed one in a 1983 123???? Drop support, and tilt trans down????

Gentlemen,
Thank you for your time.

Nick Kesseru
Spokane Wa.

Here is a link that surely will help