If your car was drum brakes and you converted to disc, let me help shed some light on the problem. Drum brakes didn't use a portportioning valve. Instead, that's a brake light switch.

I think that an adustable porportioning valve would be the only way to go. Unless someone can offer a better soution or know of a referecne, I'd adjust it, using hit-or-miss adjustments, before I'd consider allowing a professional to adjust it.