727 sometimes not going in reverse

No, the servo bore will be fine.There is no metal to metal there, and the servo is only used in reverse, and manual low, so for most of the tranny's life, it was probably seldom used.
Well you didn't complain about high gear dropping out, so I've been concentrating my thinking on the L/R circuit. There is every chance that the front clutch actually has the issue. It is even more likely. So while the VB is off, air-test that as well, but limit the pressure to 100psi. Apply,hold and listen. Now this is a tough test, cuz you have nothing to compare it to. This test always produces a hissing noise, as the cast seal-rings are lousy at sealing air. So you have to put on your thinking cap, and picture in your mind how it works. When OIL enters the circuit, it pushes the big piston out to clamp the clutches.Until the piston stops moving, this is a low pressure circuit. Once the piston stops, the oil pressure rises to clamp hard. The oil pressure backs up and some of it leaks out the seal-rings. No biggie, cuz there is lots of extra oil flow to maintain adequate pressure. That's how it works with oil.
Now with air, it's a little different.The second the air enters the circuit, it immediately starts leaking out the cast-rings. You will hear it blow the oil out. It sounds like a kid blowing his nose.Then the pressure builds up and blows the piston out to it's limit and it makes that nice thunking noise. Now the pressure backs up and really starts to hiss as it leaks out the rings.As the pressure in the piston rises, you will hear the hissing change pitch, and finally stabilize. At that point, you can release the pressure, but keep the rubber tip in the hole to prevent air leaking out there. Keep listening.As the spring forces the piston back to it's parking spot, the air will continue to leak out the cast rings.
Here's the tricky part.If the cast rings are bad all the air in the circuit will almost instantly be dumped.But if the rings are good, it will take maybe a quarter to a half a second for the air to blow out of the circuit, and it will sound like the last bit of air escaping from a balloon. Kindof a sigh.You will have to listen for the sigh.
Bad seal rings are not the only thing that can cause instant pressure release. A miss-match of parts in the front can do it too, as can improper washer locations or excessive end play.For this reason I always set up the complete front end on the bench and pressure test it through the front housing.By the time it gets installed into the case,I have every confidence that it will work.
It may be that in the distant past somebody waited too long for 2nd gear band adjust, and burned up the front drum. If someone replaced it with the wrong part, the tranny would have had this issue from day one after the rebuild.
Lucky you!