Gmc safari brake issue

Hi, I have a 96 GMC safari (Geneva). She just took me for a 17,000 mike road trip! Without a single problem. In California I nearly destroyed her bc I had no idea about engine breaking and I road the hell out of her brakes. So I replaced the calipers rotors shoes lines and fluid. Had a great trip on them cross country back to the FL keys. Since back I have had trouble with the brakes not functioning. Stiffness in the brake pedal and not reliable braking. Took her in recently and had both front ABS Boosters and sensors replaced. Now the brakes are holding and I could smell burning rubber after driving home from the mechanics. My mechanic has no clue what’s going on and is suggesting I out new calipers on but is unsure that’s actually the problem. I’m confused lost and need advice I’m no mechanic but I would try not to seem so confused when talking to someone who just spent a lot of money at my shop. Bringing her for a second opinion today. Any and All info is appreciated!!??!!

If the rubber lines have never been replaced they can collapse internally. They act as check valves not letting the pressure off the calipers when the pedal is released.