"My '88s brakes start out weak. If I drive for perhaps 20 minutes, the brakes become awesome."
Running on highway etc won't affect Vacuum problem to/at/in the booster.
Running on highway can/will heat the pads. Worse if the pads are dragging a bit. (If your pads are dragging because of caliper problem, that can/will cause brake fade.)
My guess you have "racing" pads that needs more heat to work.
Normal, like any "cheap" pads from AZ etc, and Premium pads, like Wanger TQ, pads won't do this.
Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
The Ogre's Fiero Cave (It's also at the top and bottom of every forum page...)