Trying to figure out my brakes

A couple great suggestions guys, but first thing I checked for. I also did find a funny little bracked sticking up from the firewall under the master cylinder.
Sometimes it can be fun dealing with 38 year old vehicles, you never know where they've been or who messed with them.

OK . Vacuum boosters do have added support brackets. One underneath and one to the left inner fender. Installing a booster might be the cure.
I would first find out if there were different master cylinders for power disc and manual disc. You may get by with just a different master cylinder. Good luck