You need to check some pressures. Did the trans just start doing this or is this a fresh/rebuilt trans? The T-brake only applies Rev. with 1st gear slipping the car should back up. Checks pressure at the servo cover and at the reverse tap (if you have one). Post your results. You can give me a call at the shop to 727-375-4737 Matt
Also if this trans just start to do this, I have had a few hipster aluminum brake have the bolts come loose.