Help with transbrake not holding!!!!
 

I have a Coan powerglide with a transbrake in my 84 GT. Last week when I would stage. I pushed the transbrake button and when I hit the gas to bring up the RPM's to my launch rpm the car goes forward several inches then stops and when I let off of the button it launches. It did not do this before. Has anybody had this problem and how do I fix it. One guy told me I need to adjust the bands in the tranny. Any help or info would be greatly appriciated.