My rear brake prop valve leaks and my brake man can't find parts or replacements. He says he's never seen such a setup before. Is there a reasonable replacement system such as from a minivan or maybe a K-wagon that can accomodate the load variations of a pickup? I currently have the complete brake system from an '85 Shelby Charger in place. I'm having new rubber lines installed from frame to hub to replace the old swollen units. But the prop valve is a problem I can't just remove as this is still a 'work truck' meaning I do haul the occasional load and I don't want any squirrely performance in the rear while doing so. Any help would be appreciated here.