Are the valves to which you refer on the coach or the toad (in the Brakemaster)? If the proportioning valve is one that is part of your coach chassis air brake system, why not just disconnect the toad bake line for now? You won't stop quite as fast, but any large diesel coach should have no trouble stopping a toad.
On the other hand, what's thr problem if the coach air compressor is running a lot? It cycles all the time anyway and should have plenty of excess capacity. Wait a minute: Is this perhaps a gas chassis coach with the add-on compressor exclusively for the Brakemaster? That's maybe a different story...
By the way, do we know each other from years ago? RVForum?
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL