Expired Polaroid 664

I recently picked up 3 packs of black and white poloroid 664 (pull apart). The chemicals are all dried up but I decided to try and develop the negitive in xtol for 8.5 min and stoped and fixed for the normal times and to my surprise you could see the image. However, I couldn't remove the remjet on the back of the film. I tried warm water and also the gel bleach. Any other suggestions on how to remove the backing ? Thanks in advance.