I've got a beater 1984 300D that I haven't driven regularly in a while. When cold, it shifts just fine all the way up to 4th, although maybe some of the upshifts are a little harsh. Once it gets up to operating temp, the shift points seem to rise unreasonably high, to the point where it never gets out of 2nd when driving around town. It will shift into 3rd if you get it going fast enough (like 5000 RPM), but I haven't driven it on the highway yet to see if it will go into 4th at all.

The fluid level is fine, the fluid looks good (red and non-aerated), and it doesn't smell burnt. Adding a bottle of Lucas Transmission Fix (which has given me good results in other marginally-shifting vehicles) made no noticeable difference. The transmission does have a slow leak, so it's possible it was run low on fluid at some point, but I don't recall it having this behavior immediately before I parked it for a year.

I'm hoping the interior guts are still good and there's just an adjustment that needs to made somewhere (vacuum modulator, bowden cable, etc.). Unfortunately, everything I'm reading about this transmission is pretty complicated and I'm not sure where to start. My current plan is to make sure I still have healthy vacuum to the modulator, but if that's not the problem I'm not sure where to go from there.

Any suggestions?