Power brake conversion issue

I recall a 88 T-bird that would begin each day just fine then keep enough residual brake pressure to cause hot lining odor. The lady had the lining replaced a few times in a short period, front calipers replaced once also. Her son ( one of the salesmen where I worked ) said he thought she drove with 2 feet, riding the brakes. When it came through the door I couldn't turn a front wheel by hand. The next morning I could. I did suspect the booster but found the master cylinder was the fault. The spool valve/piston/whatever you wish to call it would stop just short of fully returned, as if the spring inside was weak.
The front wheel drive Cutlass Ciera that would lock up only 1 rear wheel is still the most notable oddball case in my memory. That was a pre antilock master cylinder proportion combo assembly.
I don't know that the T-bird came off the assembly line with a flaw. It had been in a fender bender.
I feel certain the lil Cutlass had its brake system flaw from day 1. Not all brand new parts are good parts. Good luck with yours.