I've got twin Dexter 5600 lb axles w/12" x 2.5" electric drums (trailer tagged as 9800 lbs GVW). I service the brakes a lot but there is always some problem. I'll make the brackets, add the calipers and do whatever machining is needed to mount rotors. But its the electric to hydraulic servo I'm pondering. (assume keeping the electronic brake controller now in use) Thought experiment: use a common vacuum over hydraulic servo. Add a 12VDC vacuum pump to provide vac source. Find a PWM vacuum valve, which would likely be an off the shelf emissions part. Adapt the PWM output from the electronic brake controller, to the vacuum valve, so the valve sees a proper signal. For emergency stop, have a 2nd dedicated hydraulic master cyl with a big pinned spring and a pullout wire on the pin. Ideas?????