Holding power in the transmission is determined by friction surface and apply pressure. So first, we need to check the pump: it has to be in good shape, nice pocket, about .002 clearance across the top of the gears in the body. A loose pump, of course, will drop pressure, especially when the fluid warms up. Normally on a pressure test, most brakes will drop from 200 – 250 psi to 50 psi and then immediately return to approximately 5 – 10% of the original pressure. The low band is on when you get the shifter in first gear. When you push the brake button, it applies the reverse clutch pack if the piston is pretty large, so the fluid requirement can be big, especially if the piston travel is too much. Normally, we will set the reverse pack at .050 - .060 with a 5-6-7 clutch pack. Of course, the more clutches, the more holding power, but also the more drag. I think that you probably have a pump issue.
You can also email Mike Stewart at email@example.com