Your picture shows a standard Bendix R-12 relay valve (with 4 port) and a pressure regulator (it doesn't permit air to pass through until a preset pressure is reached, usually 65 or 95 pounds) dangling by nylon tubing. I'm sure the howling you heard was your rear brakes engaging as the parking brakes began to set up. No low air buzzer? Just lights and a gauge? Why wouldn't the manufacturer just tee off your existing relay emergency valve? Was that R-12 added by your toad brake supplier? I carry an assortment of tubing fittings in various sizes so I can cap a line if necesary so I can get off the highway and to a service provider.