The other cause of "soft" braking and my first thought - broken strands in the wire, usually inside the brake lever. Since that can be the cause and leads inevitably to more, then all the strands breaking, finding the cause of "soft" brakes is one of those really important matters. Front brake? Ignoring that feeling could have you in the back seat of some stranger's car. (Breaking and entering through the rear window isn't pretty, even if the owner doesn't press charges.)

Ben