where am I going wrong?

It sounds like the over center spring is STILL THERE, and he said the magic word, "diaphragm" (snicker).

If it's a diaphragm clutch, put your car on a diet and start with tossing that over center spring, It's not used with diaphragm clutches.

How sure are you that your z- bar/pivot shaft is correct, not installed backwards (if that is even possible), etc?