There are two vacuum lines going to the trans. One goes to the modulator. IF, that line was broken your shifts would be real hard.

If during the time it was shifting hard it had the modulator pressure reduced to soften the shifts and then someone found the leak the natural effect of solving the leak would be to significantly reduce modulator pressure in light load (high vacuum ) conditions.

I would start with modulator adjustments.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician