Supposedly the hydraulic to electric controllers were discontinued because of antilock brake systems.
Yep, I had a Kelsey-Hayes modular hydraulic to electric brake control that worked perfectly for years. It had a tee at the master cylinder with a hydraulic line to a sealed electronic module mounted on the firewall near the master cylinder. A wire harness from the module connected to the controller mounted on the dash. I towed trailers for thousands of trouble free miles with it. I have never liked the electronic inertia type controllers and I miss not having the K-H controller.
I'm seriously considering the Maxbrake controller like Larry Willard is using. When I do, my Prodigy is going into the dumpster!
Bill