That almost looks like an accumulator spring. I bet that spring slowed down the application of the band and I think that was your original complaint.
I suppose,that big fat spring wrecked the thin tin retainer first. Then if the servo came up too high,(not likely, but possible),it couldda spewed fluid out the top and there went your clamping pressure.If this could or did happen, very likely the servo would have wrecked the lip seal on it's way back in.And that would lead to slippage of the band as well. If it was me I would do an air-pressure test with the VB off, trying to prove that. And if I was not satisfied with the test, I would pop it outta there for a look-see.
I think I have replaced that spring with the VB on, but had to dislocate the apply strut to do so. In your case I think I would drop the VB and inspect the accumulator spring, and compare it to this blue monster.
Alternatively, maybe someone will chime in who is more familiar with the TF than am I, (which shouldn't be hard,lol).
