What is the most common cause of a 3-4 flare? Also, does the flair damage the transmission?

Have you checked throttle linkages, Bowden cable, vacuum at the IP, modulator, etc? Have you messed with the modulator on the transmission at all?

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)

I think all linkages are fine. Not sure about the vacuum connections though. I don't even know if the vacuum part is connected correctly.

I agree it may not be a catastrophic failure even if you are describing one to the T. Youre describing transmission slipping, where basically the Engine RPMs dont match your acceleration--like its overrevving and not moving as such. Try to have a competent ford tech or independent mechanic flash the trans Control Module or P(powertrain)CM. If it ends up being the transmission, then id try some stop-slip by Lucas Oil or something of the sort. Nothing to lose and its actually helped me before.

Listen up! Those "ford" guys really know their stuff when it comes to an MB transmission.
badtime indeed.

Where, exactly, do you find a PCM or TCM on a 1982 300CD? (I am thinking you need to have your module flashed.)

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

"Flash the trans control module"? Yeah, I'll get right on that!!!

https://www.reddit.com/r/cars/comments/jq3avj/comment/gbjetnp/?utm_source=share&utm_medium=web2x&context=3

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1981 240D 4sp manual. Ivory White.

It being Saturday, there is a risk of being a spoil-sport by returning to the OP's question.
Herewith some attempts at answering:
The 3-4 shift involves fluid flow to two operating elements (Band 2 release, and Klutch 2 apply) & and one accumulator.

So:

1) The output volume of the pump must be sufficient to supply these three simultaneously.
2) The working pressure (modulated) must not be too low.
3) The leakage past the seals of any or all of the three must be at a minimum.
4) The friction material of K2 (Klutch) must not be excessively worn.


The flair occurs when band B2 releases faster than clutch K2 applies.
A flair that persists causes rapid wear of the clutch friction material.

Replace the k2 spring with the updated version worked several times for me.

__________________
92 e300d2.5t
01 e320
05 cdi
85 chev c10

Your cure for that is popping in a new "K2 Trans Piston Kit" Part No. a 126 270 05 35

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'83 SD, 2x '85 SD
You are entitled to your own opinions, you are not entitled to your own facts.

S0, I'm gathering that the "K2 Trans Piston Kit" Part No. a 126 270 05 35
is the fix?

Can anyone tell me if the vacuum connections will also help? Post #3 has a pictures of the connections.

gp:
As a first, and quick, check for possible working pressure and vacuum modulator issues, disconnect the vacuum line that goes down to the modulator on the transmission (or disconnect it at the modulator). When driven without vacuum to the modulator the working pressure will be at its greatest. If the supply of vacuum to the modulator is a problem, the flaring will be reduced, if not entirely eliminated.

Thanks!!! I'll check that. Can you confirm which vacuum line should be connected to the modulator?

I’d recommend checking the vacuum diagram on your radiator support underhood and/or the FSM.

__________________
Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (116k)
1991 350SD (206k)
1991 300D (228k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (267k)

Past Diesels:
1983 300D (228K), 1985 300D (233K), 1993 300D 2.5T (338k), 1993 300SD (291k)