I've replaced the master cylinder and the brake pads, but te brakes remain overly soft. What could the problem be?

My 91 is doing the same thing as far as i can tell its got to be the master cylinder or brake booster or weak brake line

Check front calipers for leaking fluid, remove the back brake drums and check for fluid leaking from the wheel cylinders, if all that looks fine you have air in the lines somewhere they will need to be bled, start with the wheel closest to the master cylinder and work you may around to the one furthest from the master cylinder.

Also check your rotors and drums for minimum thickness, if they are too thin you may have pushed the caliper/wheel cylinder pistons out to far and sucked air into the brake lines.