Does your C6 have a valve body detent or transmission case detent? And finally, is your throttle valve (vacuum modulator) screw-in (before '72) or press-in ('72-up)?

That's where the vacuum modulator comes in: A rubber diaphragm located inside the modulator housing on the side of the transmission case is referenced to engine vacuum.

The C4, C6, and FMX transmissions employed throttle valves, or vacuum modulators, to control shift points. Based on intake manifold vacuum, throttle valves adjust transmission control pressure as ...