The basic operation of the hydraulic release is shown in Figure 2 of the Bendix TW-12
Service sheet.
The difference is that the Alpine uses a TW-11 control valve and the HR-1 relay operates a single transmission drum brake not the 2 axle brakes shown on the diagram. The Alpine also has a pressure switch connected to the TW-11 that operates the parking brake lamp on the dashboard. In fact I thought I had a leak in the control valve but it turned out it was the pressure switch that was leaking due to corrosion, so I would start with that.