1983 300SD.

The transmission shifts somewhat hard 1-2 and 2-3. Not too bad, but somewhat hard.

But from 3-4 there is an excessive delay and engaging in 4 feels spongy. When I am accelerating fairly hard, the 3-4 shift feels appropriate. I have a 78 300CD for comparison, and it's shifts are smooth and lovely by comparison.

When I first got the 300SD, I replaced the 2 plastic levers on the vacuum valve. Also, I recently had a fluid and filter change on the transmission and torque converter, and had the mechanic look for any signs of trouble--and he said it was fine, as far as he can see.

Now I wonder if the whole vacuum valve is suspect, or if it's the 3/2 vacuum switch, modulator valve or it's something else (I plead ignorance of all things automatic transmission), or if I am soon due to spend $$ on an entire rebuild.

Any thoughts are greatly appreciated.