Andrew R Stewart 

I didn't read of anyone mentioning that the middle ring falls into the shadow of the large and small ring teeth. meaning if you place a straight edge on the tips of the teeth on the rings, crossways to the frame, that the edge will likely only contact the small and large rings. When the chain is pushed to the outward direction from the small ring it will tend to catch on the large ring before it touches the middle ring, hence being lost in the shadow. Wider ring spacing, pinned and ramped rings and a ft cage with a deep inner plate all help reduce this. But making sure your middle ring sits above the small/large ring "slope" helps a LOT. Andy

Eggman84

Good information Andrew; never thought how the middle ring could be in the "shadow" of the small and big rings. But I find something odd about the OPs original post. There text is in italics below.

I converted a perfectly nice vintage Trek triple crank bike to brifters. I assume that this bike originally used friction shifters (what I would expect on a vintage Trek).

The only fly in the ointment is the shift from granny to the middle chainring. The chain just grinds against the big ring which keeps it from going to the middle. I would think that if this is happening with the brifters, then it also had to happen with the original friction shifters. Did the OP have to"overshift" with the friction shifter to get the chain to shift to the middle chainring, but with friction shifters it is a "smoother" operation to overshift and then adjust. With brifters, overshifting and then adjusting (or trimming) is more obvious because you have to use two separate "paddles".

Yes, brifters can be finicky, but if the chain is just grinding on the large chainring using the brifter, and the only thing the OP replaced was the shifter (no changes to the FD), then the chain must have been grinding on the large chainring with the original shifter. Or am I missing something?

As to solutions, agree with recommendations by others.

Cycle Tourist

No. Your not missing a thing. I tried different derailleurs, different cranks, different angles and heights. I got mixed results but nothing satisfactory.
I changed back to a friction down tube shifter and had the exact same problem. I dissembled the bike when I first got it and belatedly realized the problem was there all along. 😩
Another lesson learned. The brifters probably would have worked fine had I fixed the initial issue. The chain looks very laterally flexible and I think/hope a new chain will address the issue.
Any thoughts on what chain might be a good choice? The shadow comment is interesting too. The Maxy triple and Cyclone derailleurs were original equipment I think!!! so I'm hoping the new chain will finally allow me to get this old TX700 listed and sold.
....older and wiser.

Cycle Tourist

No your right on. That is the last time I'll year down a bike without riding it. Assume at your own risk.😬

Cycle Tourist

I get that but usually I have no issue using old derailleurs with indexing shifters. Suntour and Shimano 7 speed stuff works interchangeably. Smarter people than I have told me it's just lucky that it works.

Unca_Sam

Could you provide a link to the levers?

Cycle Tourist

Thought I'd post the final repair since everyone was so helpful. Your observation wins the award though. I tried alot of stuff but a smaller, big chainring did the trick. A 48 instead of the 52 that was on it. The chainrings were Sugino on a Maxy crank. I almost gave up and put a double on it but a Trek TX700 deserved a triple. Thanks for everyone's suggestions. This was one I hadn't come across before. I had a recummbant touring bike that had a 53-42-24 and had no problem but I was probably just lucky I guess. The looong chain probably helped too. Thanks again.😘

Eggman84

Thanks for posting the solution to your problem. And thanks to Andrew for suggesting the potential solution.