727 sometimes not going in reverse

Sounds like I need to dig out my pressure Guage. There is no gasket between the valve body and transmission on a 727 though. I think you may have your order of operations mixed up with something else as well or we are speaking different terminology. Reverse is the same as low except reverse uses the front clutch.

My understanding:
Low-rear band and rear clutch
Reverse-front clutch and rear band
Second - front band and rear clutch
drive-front and rear clutches

I will say that after I rebuilt it I tested and found this issue, dropped pan and checked the rear band, servo and readjusted the band (2 turns off of 72 inch pounds).

Also I have a pretty sketchy shifter in right now that does not have any notches for each gear, it is essentially a lever on a cable, I typically count the notches I feel to get in/out of each gear.