I have a problem with the transmission of my 1981 380SE. Transmission type 722.3.

The car starts in 1st (which is a surprise to me) and changes into 2nd at 5-10mph. All gear changes are fine except 2-3. When accelerating gently the change is ok-ish. But when I accelerate harder, the box flares and gives the impression of slipping into neutral. It will not engage 3rd unless I stop pressing the accelerator and let the car glide along for a bit to slow down a little. 3-4 change is fine.

The fluid and filter have been changed as per manual and the level is correct.
The fluid which came out was not new but didn't look too bad. There were no pieces in the pan.

I've read about the B2 piston in these transmission types. Could I be experiencing a failure of the B2 piston here?

Thanks,
Adam