Bob's Studebaker Resource Website Studebaker Hydrovacs
This is a Type A Drum Brake Hydrovac
Bob's Studebaker Resource Website
Here's how it works
As pressure builds and the diaphragm is pushed even further forward, the atmospheric valve portion opens (shown in purple) allowing air to enter the power cylinder (air path shown in green) and letting the vacuum from the engine pull the power cylinder piston (# 2) forward which pushes the hydraulic brake piston (# 20) into the hydraulic cylinder (# 22). The check ball has pressure from the brake lines pushing back against it now (orange) and seats inside the piston allowing no brake fluid to pass by it back into the master cylinder. Therefore, the piston pushes more brake fluid forward in the lines and braking power increases.
The whole system uses brake pressure to overpower the springs and engage / disengage the control body valves. The reason this system works the way it does is that the vacuum created by the engine is able to travel throughout the Hydrovac unit (shown in red). Therefore with vacuum in front of and behind the power piston, there is equal pressure on both sides. So until the valves close off that circulation and opens the air intake, the power cylinder piston won't move. Closing one valve stops that circulation throughout the unit. Opening the intake allows the cylinder to pull air from behind so the vacuum can pull the power cylinder piston forward.