A '91 likely already has the upgraded "v6" brakes. Look for the step in the booster to verify that its a dual-diaphragm booster and a "1" somewhere in the master cylinder casting.




As for your brake problems, I'd be looking for brake pad/shoe contamination by oil or brake fluid. I'd also check the rear prop valve rod adjustment as kewlynx mentioned. And then the last thing might be that you have either some cheap pads up front, or some hi-performance pads that need to be warmed up to work well (like carbon metallic pads). OEM pads are generally the best, but if you want an upgrade, Hawk HPS or Raybestos Brutestop are pads that have higher friction properties, but don't need to be hot to work well.