Don't mean to just jump in but, apply vac manually to the lockoff if you haven't, you should have tank pressure. Is the button stiff on the converter? Sounds stupid, if so replace it. I was around when these trucks were new. Parts are so cheap I would just change all three, lockoff, converter, and carb. I hate going back. If no vacuum continues, these Mazda are the only engine I have even seen that the valves tighten up on. Leads to poor vacuum and ultimately no compression. Seen guys condemn engines over valve adjustment. Let me know. Kiss it!