Before proceeding, check that your rear shoes are not worn out and or not adjusted.
Per question 7, it is possible that the reaction disk has fallen out. It is a rubber disk about the size of a quarter and 1/4 inch thick and goes between the booster and the MC push rod. They typically fall out when people change the MC , try to adjust the push rod, pull the push rod too far out of the booster such that the reaction disk is pulled out of a pocket it sits in and falls to the bottom of the booster. Symptom: pedal travels almost to the floor and then the brakes grab really hard.
Recommend that you search HybridZ. There is a post by a member that details this problem. It may be in the FAQ bake section or in the brake forum.
Do your research and before doing the next step.
You will need to visually confirm that the disk has fallen out.
1. You do not have to remove the MC for this.
2. Remove the nuts that hold the MC to the booster.
3. Do not remove the brake lines from the MC. Just pull it away from the booster and push it aside.
4. Remove the spacer that goes between the MC and the booster.
5. With a flashlight, look inside the booster where the push rod is. You will see a star shaped "keeper" that holds the push rod seal in place.
6. Carefully remove the keeper so you don't bend it. You may have to reshape it when reinstalling it.
7. After the keeper is removed pull on the push rod until it comes out. The seal will come out with it.
8 With a flashlight look at the pocket in the booster where the push rod sits. You should see a black rubber disk. If all you see is shiny metal pocket, then the disk has fallen out.
Now you have a choice:
1. Remove the booster and shake it around until the disk falls out or
2. Buy a new booster as it will have a disk installed. Note if you pull the push rod out too far while adjusting the length per the FSM then the new disk will fall out.
Recommend that you glue (JB Weld) the disk to the push rod, even on new units. Just orient the booster in a vise with the push rod pointed up vertically. Dig out the disk , glue it to the push rod and then replace the push rod, seal and keeper in the booster. This will save your mental heath when adjusting the push rod length.
Your Z is over 40 years old. In the interest of safety, I always completely rebuild the brake system on old cars before putting them on the road.