[Adger Smith]

In a P/G the band is applied when it is pulled into low gear. Rolling forward is usually too low line pressure at the time you apply the Trans Brk or too much clutch clearance in the rev clutches. The low(forward band) is applied and the rev clutches must apply to hold or lock up the forward movement. If they(rev clutches) are late or slow applying you will roll/creep forward.Try changing the spring pressure or the size of the boost valve (.488 dia gets the most pressure) in the valve body to raise the line pressure. You might also try going to a higher apply RPM to get the pump spinning up the pump pressure-Vol. Low vol. or worn pumps require higher RPM to move the required fluid vol to apply the rev clutches. The hardest thing to fix is the rev clutch pack clearance. (requires complete transmission disassembly)The more clearance, the more fluid vol is required to apply the rev clutches. I usually work around .060 for pro tree and around .085 to .090 on .500 tree applications. With it doing it hot I would put a pressure gauge on the rev & check the difference in hot or cold pressures. You can also judge how fast the pressure rise is when you apply the Trans brake.
Good luck, hope this helps you solve your problem.