Does reverse work? I'm no expert by any means but when I was in high school I rebuilt 12 727's-904's in the vocational auto mechanics class I took in my senior year. In fact my first car was a 71 Demon with a bad 727 and the teacher had me rebuilding it the 2nd week of class. Great teacher cause he had no lesson plans...just bring it in and tear it down. EVERY single one of them had the same problem which was the piston seal for 1st/reverse had failed. Over the years people rebuild these with all kinds of non-OEM parts including throttle kick-down cables instead of the stock linkages so who knows what the problem could be but just my 2 cents.
