'84 300SD with a rebuilt tranny, 722.3, running well for a year. The transmission suddenly lost power in all gears, including reverse. Revving the engine at high rpm got it to move a short distance then stop again, no power uphill. This was a few hundred miles after changing the B2 broken piston, which had been driven broken for several thousand miles. A preliminary symptom was a whinning sound as from a bad torque converter. Thinking the fluid was low and trying to get home the fluid got overfilled for 50 miles.

The ATSG transmission manual gives several causes including a failed main pump, torque converter or shift valve housing. Since I had the orignal tranny, in good condition except a failed K2, I had a source of spare parts. So I changed out the front pump, torque converter (not changed when substituting the rebuilt tranny) and valve housing. I did not check the modulating pressure control guide, another listed cause. Upon reassembly and try-out the transmission it worked normally for a couple miles - torque, up-shifting, down-shifting etc, then suddenly lost power in all gear positions again! Letting the car cool a few minutes got it going normally again. I got it home and parked, now I'm very puzzled.
While working on the substitutions I examined parts for signs of wear, chips, burt fluid etc and everything looks good.
Any transmission experts out there with some hints about this? I'll be removing the modulating pressure control guide later today to see if its the culprit but I'm astonished at the moment. Any guidance is appreciated.