so I blew reverse again.

Hey Guys, my thoughts are iv got a valve body or shifter problem, or im assembling the tranz incorrectly.
I pulled the tranz out a month ago and found it had pulled the friction material off the rev/low band and scored the drum badly.
I replaced the drum and got a TCI band from summit, I put it all back together, rechecked end float and put it in the car.
I went for a little test drive(5miles or so) after filling it with ATF,
I stopped at a shop for 5mins and on attemping to reverse out of my park, I found rev was slipping.
by the time I got half a car length out of the space it wouldn't move below 2000rpm so I stopped and checked the fluid, fluid was full.
I pushed it out and drove it home, by the time I got home rev is gone entirely.
I noticed a light drag in the car as if I had the brake on slightly and now that rev stopped working the drag is gone.
can anyone help here? anyone seen or heard of this before?
the first time I built the tranz the low band was second hand but this one is new! what the???