727 sometimes not going in reverse

The brand new band, doesn't automatically guarantee that it's any good.
No; I'm referring to the VB gasket that covers the ports that you air-tested in the accumulator piston area.
As to case issues, that is highly unlikely, but Ima thinking porosity or a crack, or a bad/cracked servo.If you are confident about the air-pressure test results, it would automatically rule all that out.All being; the gasket, the passage,the servo, and the case.
I should also mention that in reverse, the maximum pump pressure is directly routed to the servo and the Hi-drum, via the manual valve. The pressure regulator is out of the circuit.There are no other circuit branches involved.With maximum pressure,the only thing preventing reverse from slamming in, is the cushioning built into the Hi-drum.Most guys take that cushioning out for a slam-bang 2-3 shift.What I'm suggesting is, there should never be a hydraulic issue engaging reverse.If the oil-pressure test shows that, then,with all the info you have supplied,I'm already suspecting the band or the band-adjustment.
So oil-pressure test would be next.