Is your torque ball and transmission tail piece socket polished smooth?
If so, did you test the amount of effort it takes to move the ball without the gasket in place? This shimming procedure is a pain due to all of the assembly/disassembly/reassembly, but it is the only way to keep them from leaking.
They all will develop leaks over time, but it shouldn't leak from day one after you properly reseal one.
Carl
